One word: froogle. (OK, actually froogle.google.com. Put in your search term, search, and then the key: select "sort by price: low to high".

(Another key: make sure that the description specifies that the shorts are padded! I found a lot of shorts for under $20, but most didn't mention padding. No thanks!)

Other than that, some people may disagree with me, but I have yet to find a reason to stick with women's cycling shorts. If you find men's cycling shorts on sale, go for it!