Look at the clothes involved before committing to anything. I mean actually look at them, not pictures of them or descriptions of them. On top of that, if they are very stretchy or worn, you will need to stabilize them. That often takes more time than anything & isn't particularly fun to sew through. You also want to note the fiber content. If they are all similar fibers, it's not terrible. If they want knits mixed in with satin or velvet or rayon & such, it starts getting much more complicated.
After you've decided if it's worth taking on at all, add up the cost of everything -- not just the basics, but the cost of things you "just use a little of" like needles, thread, laundry detergent, markers, starch/sizing, electricity, wear & tear on your machine, and so forth. Then, do your best to come up with an honest estimate of how long it will take you to do every single step of construction. For me, that's: pre-wash, iron, stabilize as needed, cut, pin, piece & add on borders, mark quilting design on quilt top, layer & pin baste, quilt, bind, add label, wash twice & dry. If you're going to have to buy the stuff, remember it does take time & gas to go shopping ... and if you're "just using stuff from your stash", I would still charge at least 50% of what you originally paid because it still isn't free. Honestly, it depends on the market you're in. Some markets support making $10-12/hour for your time. In other ones, you'll be lucky to get $2/hour. I usually encourage people to charge a livable wage, but at least you don't live in the Midwest (by me) so if you want to work for the wages of someone in a third world country, go for it. Just know what you're getting into before you start.
As you can probably tell, I learned these lessons the hard way. Spent 50 hours on a quilt that I got $50 for (not even when you consider costs of thread & other such "small" items). Usually, on an applique quilt that I spent 50 hours on, I'd make $175-200 (that's my pay after I've taken care of all my costs) so I was just devastated that this "simple little quilt" that I thought was going to be some easy money turned out to be a financial disaster. The fabric wasn't "like new cotton" as I was promised, but hole-riddled poly-cotton that was nearly thread bare. The client did buy the main materials (border fabric, backing & binding, plus batting), but there were all those other little costs & way more time than I would have ever thought. But as I said, if it's all t-shirts that are in pretty good condition, it wouldn't be nearly so bad. Just make sure you're going into it with eyes wide open.
