Is there much of a price difference between 100% cotton and bamboo/cotton blend?
Yes. Unfortunately, the bamboo is a lot more expensive. I think the list price for the Queen batting was $39.99, but I only paid 60% of that because I had a coupon.
I've heard that one of the drawbacks of bamboo is that it doesn't wear as well as cotton. I don't know whether this is true or not, but it's probably something to consider when making a quilt. (I suspect it's true based on my experience with bamboo linens from West Elm.) I'm not someone who makes things with the idea that people will still want to have them 100 years from now, but if you are creating "heirloom" items, that may be something to consider.