3 man band (bass, drums, vocals-guitars)... want to bring Sample player/sequencing for fuller sound without using full keyboard.. we play pop rock mostly.. 
I am good with Sonar but never explored samplers or other equipment in that nature.. any idea that i can try out.. can not spend more than $400... is Maschine Studio an option?

Does "no keyboard" mean canned sequences you'll play along with? If you've never done that before, I can tell you that it's frustrating as hell to be confined to prefab arrangements. If the crowd is digging a song, you can't stretch it. And if the song's not going over, you can't change your mind midstream and launch quickly into something else.
 
But if your set list is rigid and your show is tightly scripted, it can work. If you do go that route, you won't want to use a MIDI-driven sampler onstage, but rather prerecorded audio files. That'll give you the fastest setup time between songs, and you'll only need an iPad or something to pull it off. Instead of a laptop, which may not be rugged enough for the perils of travel, use a tablet computer on a mike-stand mount.
 
As far as recording those backing tracks, the sky's the limit as to what you can use. For Top-40 covers, Kontakt is probably your best bet because it'll handle strings, brass, percussion, synth pads and leads - pretty much anything. That'll fit your $400 budget.

I've never used Maschine but it may be an option if you're looking for sample/sequence triggering with the flexibility to mix things up on a whim. The Maschine software works around "scenes" - there are many YouTube videos on the subject which could help you decide if it's the kind of thing you're looking for. In terms of a controller though, Maschine is top notch. There is also the Akai MPC to look into, Akai is probably what most people think of when it comes to samplers and sequencers. I'm sure if you put this question out on Gearslutz you'll have a trillion opinions within a couple of hours, lol....

Also as a FYI Akai have just released the MPC Touch which looks very nice. But over your $400 limit unfortunately.

Boss Loopstation should do it for you. You can control it with your feet as well. Also, you can route your guitar and vocals through it, so you can use that in your performance.

I'm with Dave (Bitflipper) on this.  With a limited budget especially, using pre-recorded audio files are the best, cheapest, and most reliable way to go.  In addition, there's way less gear to carry around, set up, and troubleshoot.
 
Use any good portable audio player... high quality MP3 or Wave.... Ipod, Tablet, or laptop.... and simply plug into an open channel in the PA.
 
I saw a band once, comprised of 2 guys. They had as much gear as a regular 5 piece band..... racks of sound modules, foot switches, amps, etc.... everything was run by an old desktop computer running an early version of Cakewalk midi sequencer. They sounded good, but I can't imagine the mess if that computer had glitched in the middle of a song with the dance floor packed. It's always best to go with the most simple and reliable gear as possible. 

In fact.... if I was doing this now.... it would be a laptop as I described above. HOWEVER.... I would have TWO of them..... with the primary one running the show, and the second one turned on, plugged into a spare channel in the PA and with a duplicate of the show music software running in the event of the primary one crashing. Pull one fader down, slide the other one up, and click on a song. In 5 seconds or less,  you're back into the music.

Get a keyboard player. And hopefully (s)he'll show up to the gigs.

We're a 4 piece band (2 guitars, bass, drums, 3 vocal parts) that go out with backing tracks to take care of any keyboards, orchestration, choirs, or any times we need to replicate a big wall of guitar harmonies (a la Queen) that we do on our albums.
 
Basically it's all on an iPod, which we split into 2 channels: Left channel goes to the PA with all of the backing track elements (usually via a DI box or a small format mixer next to the drummer), and Right channel goes to the drummer's headphones which has a metronome for keeping time in the song.
 
We've found that we can keep a lot of the live energy up by programming songs to be a little faster than normal or extending sections, or even having parts where the click stops entirely and we play in free time as a 4-piece, and then restart the next section later when we need to bring the backing back in.
 
The only really thing you need to worry about is the drummer losing his/her place and blowing the set (that's never happened to us yet, touch wood!). I agree with what others have said here - you can tend to lose a bit of spontaneity if you're using sequenced backing, but if your set is fairly tight and you can play around the hard-coded parts, it can be really effective.