Well now I have a better feel for what you are doing with "making a tape". You are just recording their live concert via mics. No direct line-in recording. The sample songs you posted are quite nice and sound really good. They should be happy with the results.
 
Regarding your original inquiry about charging, I guess you have to decide. Was the experience of getting there, setting up your system, and sitting there for 'n' hours an enjoyable experience to you? Maybe they bought you a round or few of drinks perhaps. I guess you have to decide. But I still wouldn't feel bad about asking for a few dollars to cover gas, the 6-demos, etc. Doesn't have to be much. But then again, if this was a great time for you and you met a good group of people, well, then maybe the cost to you is 'priceless'. I don't believe everything we do in life has to have a price tag on it. At some point though that could probably change (e.g., they ask you to record every weekend for the next 6-months). In the meantime, have some fun! (Please note that my input is coming from a purely musical hobbyist viewpoint. I do understand the professional opinions).
 

tell them to buy a portable recorder.
done.
 

The last time someone asked me I said I'd do it for free except for the Bouy taxes, but they couldn't afford it...

Batsbrew....
You're comment is actually really helpful. It gave me the idea to help them make a group recording that I then load into my old Tascam and teach them to record their takes in it. Then they can pass the Tascam around each playing their takes into it. This might be a great way to get lots of takes and sufficient isolation without having them in my house the whole time.
 
We'll see. I'm not sure this group is up to the learning curve.