i agree there is a HUGE difference between (big box store) flannels and the quality heavy double sided wonderful flannels sold in quilt shops.
do yourself an experiment & buy a yard from each place & compare- before pre=washing, after pre-washing- with cutting- with sewing-
you will be able to tell the difference & decide for yourself which flannel you want to spend your money on-
when making raggy quilts sometimes the thinner- lesser flannels area just fine - they ravel- rag very well- and are ok since you are working with multiple layers- it really depends on the project -future plans = deciding which flannel to choose- just know- the differences are HUGE.
my favorite- go to most of the time flannels are Benartex double sided flannels- worth every cent!