Are you talking about the custom fabric at Joann that is special order? I don't know if it helps price-wise but it frequently goes on sale 50% off the price shown on the sample, as do their premium cottons that are available in store. You could also check eBay, fabric.com, and other online sellers. IKEA sometimes has nice fabric, if you have one near you, and they have some premade curtains that are very popular and inexpensive.

I use fabric.com often.

Fabricguru.com

You might be able to buy something ready made at IKEA that is Cotton for less for they sell their curtains as pairs (as curtains should be sold!)

I'm afraid you may not find fabric less expensive than a sale curtain from a discount store. Buying online will also likely incur a shipping charge.

Finding fabric for under $10 a yrd that is suitable for curtains, would be VERY difficult, if not impossible.

Sewing window treatments is a completely different kind of sewing than garment sewing. The terminology is different and the construction is backwards from what you might expect (you start drapes at the hem not the top). You should do quite a bit of reading and research before beginning your project. $10/yard would not be expensive for fabric suitable for window treatments. You should also consider the cost of lining and possibly interlining (depending on the weight of the fabric and the amount of sunlight hitting the fabric).