I just recently (this week!) got a pair of AD900's, so I was curious enough about your question to just have a listen right now.
Of the IEM's I have, the one that sounded closest (and it was actually surprisingly close) was my pair of RE0's. However, I use my RE0's with Monster hybrid foam tips, and my pair also have has some more foam stuffed into the nozzle than stock, both of which add to the warmth of the sound. I'd imagine stock RE0's to still sound quite similar to AD900's, and the other overall characteristics - transparency and smoothness are quite similar.
This makes me suspect that a stock RE252 would also sound quite similar, and while I have a pair of RE252's I don't want to mislead you with a completely irrelevant comparison because they are heavily modded to the point where they probably don't sound anything like they did originally.
My friend has a pair of RE-Zeros, and from memory, they sound like sweeter RE0's, so again they would probably be quite similar.
The second most similar IEM would probably be the Ortofon EQ-5, though the Ortofon is both out of your stated price range, and also is a great deal more dark in terms of sound. However if you wanted something similar to the AD900's but darker they are close.
I didn't really try anything else because I know nothing else I own would really sound similar heh.
Edited by a_recording - 8/14/11 at 9:30pm