It's all about quality and time and endurance. If you want your quilt to last then use the best quality you can afford, even if you have to save. Remember the hours that go into a quilt. It is better to have one good quilt than three or four of poor quality ones, like the ones you can buy for 65.00 that come from China, right?
Sometimes old clothes are very good quality fabric and those can be had at the thrift stores for pennies. This is just my opinion. I would never dream of critizing anyone who does what they do.
There are all sorts of ways to find good fabric. Garage sales come to mind, reading the adds etc. I myself have bought out two ladies years ago, who decided to get rid of their fabric...
As for the dollar store versus the supermarket? again it is your choice and who cares what anybody has to says, they are not paying your bills. personally, again I will always go for quality, I make sure all my food comes from America, but that is just me.