I'd say buy blackout liner, but if you scored bargain fabric for $1/yd, I'm assuming that's not the answer you're looking for. Another solution is to use other bargain fabrics you may have, or even an old vinyl "blackout" window shade that's had the spring go, as liner. If your bargain fabric is fairly dark-colored, simply use another dark bargain fabric as liner material. If light colored, use an old vinyl window shade (this will make your curtains stiff and limit your choice of styles). Or, if you have lots of bargain fabrics and light curtain material, layer your linings to have white/black/white/curtain fabric to make it thick and block sunlight. You don't want to use dark curtain fabric against your windows as it will collect heat and fade in the summer. Another option is to buy a hideous old pair of insulated drapes at a thrift shop, rip them apart, and give them new life as the liner in your pretty new curtains.