I think if you went with low cost clearance fabric or even thrift store men's shirts, scissors, needle, thread and then batting, you could do it for less than $100 actually. But it would all be hand sewn/quilted and take quite a bit of time. The most expensive part could actually be the batting. I am specifically thinking of the hexagon GFG quilts. You can print free paper ones, but do have to cut them out.
Quilting used to be much less expensive until we decided we needed all the tools, such as the rotary cutter, mat, rulers, etc. Leftover dress fabric was used along with feedsacks.
If you go buy a sewing machine, rotary cutter, mat, ruler, thread, needles, batting, fabric, etc, I would agree it could easily run $500 or more.