The best EOS primes are the L series, and they cost big money. You might find a deal on a Mark I version of a lens that has a recently released Mark II superseding it. The best cheap prime available is the EF 50mm f/1.8; light weight, loads of plastic, very sharp lens, but not USM. Another in that league is the EF 35mm f/2. I think the lower cost (consumer) primes are generally good performers, but as always you get what you pay for.

You might check user reviews for whichever lens you have in mind at the B&H website, lots of comments good, bad, and indifferent.

The great difficulty in finding a deal on an EOS lens is they're still current, they all work on the latest digital cameras, and they're all still in demand.