Out of the cheaper iem's/earbuds that I've actually owned/heard, I'd say the nuforce ne7m at the top of the list in the ~50 category. Then I'd put the yuin pk3 then soundmagic pl30 followed by the kramer modded marshmallows. The nuforce sounded pretty full and just overall enjoyable to me. I bought them for my girlfriend to go with her iphone and she absolutely loves them.
I recently lost my yuin pk3, but I really enjoyed them when I had them. Their sound reminded me alot of a ksc75, just in earbud form instead of earclips.
The Soundmagic pl-30 kind of remind me of a poor man's super.fi 5 pro. I thought the bass was pretty solid when I had the right fit, and the mids were alright (somewhat recessed like the sf5pro), and the highs were fine (although a bit grainy on certain recordings). I know those statements don't really make the pl30s sound very attractive, but I think they are a great bang for your buck type iem @ the $25 price point.
My jvc marshmallows were my gateway iems from the head-fi forums. They aren't bad, definitely an upgrade over stock buds, but they are very very bass heavy. Even with the kramer mod done, they still are very bass heavy to my ears. The mids are pretty recessed, and the highs aren't too bad. The knock that I have against the marshmallows is that no matter how much or little foam I add inside the sound tube, the overall sound of the marshmallows just seems too synthetic to me. It just doesn't sound natural like my other headphones/iems/earphones. The bass hits very hard, but everything else seems very digital to me.