I normally make them cds of songs I think they'd like.
I get my mother books, cat related crap, and "chore coupons."
I get my stepdad things like blank cds. This year I'm chipping in to get him a new computer chair.
Hmm, what kind of things do your parents have as hobbies?
Go homemade! Knitting is a pretty cheap habit, especially if you only knit patterns that need the same sized needles! Otherwise, search through thrift stores and the like. Food is usually a good gift. Everyone in my family gets a box of cookies from me and my sister.
Hmm...more ideas...how about stationary with their monogram? If you're handy with computers you can buy blank ones and design something yourself. Does your mom wear make-up? There are usually some interesting colors around the holidays, if she's adventurous. Nail polish might be a good idea if she likes or has to keep make-up conservative. It's usually only a few dollars per bottle. Or you could get her a gift certificate for mani/pedi. Moms like to be pampered, and maybe she'll invite you along, her treat!
I think dads are so much harder to shop for. A mix CD is a good idea. Gift certificate for a movie theater might be good if he likes film. Something that can be done as a family. Board games work for that, too. A lot of stores have specially packed bargain versions for the holidays.