I am just learning to sew. Bought myself a decent sewing machine, and decided to take on my first "major" project of making a quilt for my daughter's bed (she is 9). I spent almost $100 on JUST the materials (backing, batting, material for the top squares, thread). I spent probably 10 hours just ironing and cutting out the squares. I haven't even started with the actual sewing machine yet! And this is with "standard" materials...not cutting up previously worn clothing to fit.
I would guess EASILY $300 as a starting point. But maybe for someone who loves doing that sort of thing and is very accustomed to it, it wouldn't be quite as big a deal as it is turning into for me. It does require storing all the "stuff" while working with it day in/day out. And needed a place to lay it out, or clean it up every time you work with it and lay it back out each time. (Big hassle).
I wouldn't be shocked to hear anywhere from $300-$600. Really.
I bought a good sewing machine for under $350 (got a really good deal). Maybe if you are planning on doing several of these, you might want to consider buying the equipment and learning a new skill and DIY? Just a thought.