I use 100% cotton that feels thick or dense enough. I hold it to the light to be sure it's not gauzy. And I know some manufacturers that make excellent fabrics, so I also look at those as well. Waverly, David Textiles, Emma & Mila all are carried now by Walmart and they make great fabrics. Can't remember the others off the top of my head. If you go to Thousands of Bolts and look at a drop-down of their manufacturers, ALL of theirs make good quality fabrics, too.
as far as the cost, that just varies with what you can find. For instance, I found one fabric at the quilt shop in town, $11.99/yard. SAME EXACT fabric by Maywood Studio (it's in your quilt you bought, actually, and you may remember the discussion) at Thousands of Bolts sells for $4.95/yd. Sometimes at Walmart you can find a high quality fabric for less than that if they have it on sale. I never buy fabric at quilt shops unless there is a very good reason for it. For example, their fat quarters can be $3-3.50 each. Same thing at Thousands of Bolts is $1.89 or if it's a pricey fabric, $2.09.