There's a newer studio in my area that is basically new to recording, but has invested in some good quality outboard gear, mics, interfaces, etc   Basically they called Sweetwater and bought what was recommended ... but I HAVE to say what they got, was all great stuff.  I'd say besides the facility, I'm guessing they've put in $50k+ to start off for a modest facility for demos, area bands, some mobile recording, solo acts, and stuff like that.  I won't list the studio for now, or the gear, but rest assured, they're not using Reaper, $1500 worth of mics and some $500 interfaces.  They got the "serious" gear out of the Sweetwater catalog and are still purchasing as they move forward.
Their first batch of completed projects actually sound very good, they've got a great website, and seem to be on the right track.
 
This is where I have questions.  I DON'T have that amount of $$ in gear NOW nor really expect to (I'm not recording bands anymore), but have a nice treated room at home and a lot of years of recording under my belt, starting with pro studios running 2" tape. I have invested that amount over the years, but is reflected in instruments as well.  I also prefer the *home* environment now and like what I've been able to do..basically JUST with Sonar.
 
Now...
What I've done is taken some of their finished audio, and home *mastered* it JUST with Sonar.  No fancy Manley pieces or heavy duty software in my setup....JUST SONAR (well..and a couple of modest priced plugs).   I think I've been able to improve slightly on what they've done and I'd like to get them interested in my abilities.  Let me also say I've had an amount of ONLINE mastering people (supposedly with great reps and mastering equipment) do some of my own personal stuff over the years....and while I think it was an improvement, it was mainly because it was SOMEBODY else putting on some polish, instead of me....fresh ears.  However, in many cases my personal mixes weren't altered all that much, which gave me some confidence.  I've analyzed before/after as well, seeing how much things really differed and what was actually done.  In many cases it was just a modest use of just stopping close to being part of the "loudness wars" and I was pretty much able to emulate what they did, in my opinion.

Am I kidding myself by thinking I can just use Sonar (and a couple other plugs) to do this myself on other people's stuff?  I'm not going to drop $5k to run a stereo mix thru a compressor and/or adjust stereo fields.  I feel I can get >90% (of using hardware) using what I've got now. 
Last thing...I will NOT charge much if anything but only want to help this studio out.  I'd really like them to succeed and if this helps in anyway, that would be great.  However, I don't want to look like the "bedroom knucklehead" just approaching them with a computer, as  I value my ears and my experience....and think I might be able to provide some *fresh ears* in some cases.
 
Phew...that was too long to relate... but what's the consensus?  Cake's enuff??  Again...I'm NOT thinking of competing with the true mastering facilities, but trying to offer this new studio, something better than what they have now.
 
so......???

I don't think it's unreasonable. It's really much more about your room and your experienced ears than gear and plugins. When you say it's treated, does that mean effective bass traps (down to 40Hz)? Have you taken measurements of the room to know where it's being less than honest, e.g. where your resonances are?
 
I'm assuming your speakers are up to snuff and you've got a well-placed subwoofer.

I think if you've got years of listening and tweaking audio experience and can quickly get your mixes to sound good in lots of different environments, then great! It seems to me that if you took a great engineer (bitflipper for example) and stripped him down to just Cakewalk Sonar Producer... would I trust him with the recordings of my band? Hell yeah!!
 
So...
 
Right? And one way to find out if you can actually deliver is to try. See if they call you back?
 
Me, myself, I'm going out to spin tape of a great local Jazz talent tomorrow because... well, I've done it many times in the past and ... well... he called me. :-)

Agree with above posts. Good experience ear, well tuned room and decent monitors is most of the battle won. In your case; I listened to your music; specifically "My Light" superbly crafted and well balanced mix. That said; You're definitely on the right track. Wish you all the best! 

Even mastering engineers need to start somewhere. 
 
If you know the speakers and room and are experienced, there is no reason why you can't be a mastering engineer, too.  Just be advised that clients have their own ideas about how things should sound.  It is not just a matter of making things sound polished to your ears, you have to know how to make the customer happy (even if they can't articulate what they want).
 
@

See if they call you back

 
"I can accept failure, everyone fails at something. But I can't accept not trying."
- Michael Jordan

Thx Bit and all others.  Some very encouraging comments.  Fortunately I can understand the concept of the user over the items used, being the main component.  In a field where it generally is the end product that really matters, we all know of the greenhorns snubbing recording studios that don't have PT, and more so about 'desired' equipment for mastering, when it's really just the ears foremost.   I just won't brag about my [purchased new] Auratones.... that are starting to need new foam grills, due to age!  :-)~

Mastering is all about monitoring.  Good monitors, good ears, and a good room.  If you have that, you can gain experience by doing, even if you only use the tools built into Sonar, which are rather good for the most part.  
 
The studio seems like a little startup with some budget equipment and no real experience.  Not exactly competing with studios where one microphone costs 5x their entire locker.   I don't think it is out of the question to collaborate if you want to break into the game.  

I'm thinking it all goes back to the experience level of the folks running  that studio.  How well grounded are they in the basics and the more advanced parts of audio engineering, producing and in house mastering?
 
You can always take their work and see if you can improve on their "finished" product. Whether they will use your services or not will depend on them and likely how they perceive your value to them.  If you're willing to work cheap (or for free on a limited time basis) to get your foot in the door, and they turn that down, there is probably nothing you can do to get them to send you the work.
 
If you think you're at that level, to be a mastering house.... hang a shingle on the internet with a web site. Across the net no one can see the gear you use unless you either list it or post pictures of your work space. So, the thing that matters would be the results. Do post examples of before you touched the audio and then of course what it sounds like after you did your magic to it.
 
If the studio down the street uses your services then fine.... add them to your customer list.
 
I did what you did.... once or twice. I DL'd someone's song and polished it up a bit and sent it back sounding so much better.... crickets....

The standard formula for on-line mastering services:
 
1. Use a picture of somebody else's studio.
2. Build a slick web page with animations and made-up testimonials.
3. List equipment you've read about on the Gearslutz mastering forum.
4. Use terms such as "competitive sound" (which has superseded the obsolete "radio-ready").
5. Include audio samples where the original is at -20dB RMS and the "mastered" version is at -6dB with gobs of bass. They should preferably be hip-hop and EDM examples, to attract a young (read:gullible) clientele. It also helps if every word ending in the letter "s" ends in "z" instead.
6. Offer to return files in 320kb/s MP3 format, so they'll know you're a pro.