I think you can get away with cleaning the filter with water and a mild cleanser of some sort, but I believe there is no real substitue for genuine K&N filter oil.
I think its about $15-20 for the cleaner/oil kit, which, like everyone said, should last you quite a while. I actually had a K&N on my Saturn about...ohhh...almost 10 years ago, and I recently bought a used Kawaski ZZR which came with a K&N filter. Guess what? I dug up my old K&N cleaning kit from 10 years ago, and it still had plenty of cleaner/oil left! :D