Heh, to tell you the truth, the cost per Megabyte on a flash mp3 player cost considerably *ok a lot more* more than a Hard Drive based mp3 player =\. But that's beside the point, you want to know what's the best mp3 player in the 128MB range.
...Personally, I'd go with
Creative's MuVo NX series *follow link for more info* They basically have models that can store up to 128MB from what I can tell. It even sports a nice and clear LCD screen.
I'd get one myself, but my music collection is much larger than that, lol.. about 30GB big...:P.
Hence, that is why I prefer a Hard Drive based player like
iRiver's H140 series. Bulkier than the MuVo, but it has much more "techie features" AND it can hold a hefty 40GB worth of music and or other data *like photoshop* XD. It is a portable Hard Drive afterall. Too bad it burned a nice hole in my wallet, but I don't regret it! Hehe.
EDIT:
Oh! and to answer your other question... 1337 is right, its around 960KB per minute if you encoded the song as a 128kbps-cbr mp3.