I have seven combined kitchen and dining room windows and use just blinds and valances. I found two cloth tablecloths at WalMart in the colors I wanted for the rooms and as luck would have it, they were checked. I simply did the math, cut along a line, hemmed them and made a rod pocket and have gorgeous, matching valances at all seven windows. Many didn't even need hemming as the tablecloths were already edged. I think the tablecloths were on sale for $7.00, so I paid $14.00 for the window treatments.
I've used sheets many times for curtains. I've also hit yards sales with tons of curtains (usually a church) and once got a dollar-a-bag bunch of cafe curtains all in neutral tones, then dyed them all one color for my dm's large kitchen (you can use tea bags to dye a nice eggshell color). Also, if you buy those bed-in-a-bag things, many people don't use the dust ruffle - they can be cut up for a valance or small curtain as can an extra, matching pillow case. The dust-ruffles are common at yard sales, too, usually never used.
My bedroom curtains were once a worn chenille bedspread, bought for a dollar. They dye nicely, but I left mine white. I usually leave my long curtains plain in all the rooms, then get creative and switch out coordinating valances seasonally...Liz