I agree it's an expensive hobby but so are many others, like painting, golf, etc. It's totally worth the cost to me given how much I love it and it makes me happy.
About backings- I'm not too fussy about them. I rarely use yardage for that purpose. I use muslin, piece the backs, or use something I got for a great price that coordinates. I've never used sheets but that's probably a good idea. I once used an ugly fabric for my backing and regretted it. I won't do that again.
I don't shop thrift stores but many people do. That's probably a place to save some money.
We're lucky to have this hobby. I see people who have nothing going on but hours of TV watching. Ugh!