what you can afford for the purpose you need. I am not planning on my quilts lasting 100 yrs... my requirements for fabric is "is it sturdy enough to be loved and washed for 20 yrs?" with this in mind... I think Walmart 4$ fabric meets the standard, some fabric at walmart is far better quality than others that LOOK the same, feel it.
I buy most of my fabric from joanns now because of the coupons and all of the notions they have, their fabric is not created equal either! feel it.
I have just started buying online and dont like to do it, price seems high for me and I cant really match colors and of course... I cant feel it.