Using a older, manual primes, i.e. the Pentax Takumar 50mm F/1.4 with an Nikon/Canon adapter seems to be a popular choice, and you can pick up some of those lenses for 50-60 dollars and an adapter is about twenty. I would normally find you links, but I'm on a really shitty connection, so just use google and Ebay and you'll be good. Other than that, I know Nikon has a couple decent cheaper lenses. That's about all I have to say.