The best you can hope for in the $200 price range is something that will make the keyboard sound . . . the way it sounds.
Novel concept, but the dmp3 will do that. If you're looking to alter the way it sounds, then look to an effects box.
The idea of a mic pre smoothing away some of the "cold digital tone" you're refering to is mostly a myth.
The reason certain digital noisemakers sound "digital" is most likely because of inferior analog-to-digital converters and/or low resolution: For example, a jittery wordclock can impart some y-u-c-k-y artifacts commonly recognized as bad digital. Recording at very low resolutions with bad converters can sound like . . . well . . . pure shit, for lack of a better term. Robotic. Listen to something converted to 8-bit, or a low-fi MP3 stream for an example of what I'm refering to.
Bad or poorly-maintained analog gear can also impart it's own set of nasty artifacts . . think of the terms wow, flutter, and hiss.
The most likely culprit, sadly, is probably going to be your keyboard. If you plug it in, and what's coming out of it doesn't sound right to begin with, there isn't much you can do to help it out without drastically changing the sound -- in that case, you might just consider running it through a guitar amp, or an amp simulator (like a POD, sansamp, or similar). Maybe even a big muff pie.
If all you're looking for is a very subtle effect -- perhaps adding some fuzz, or a bit of a low-fi effect, then a cheapo tube preamp might be what you're looking for. ART Tube MP, Studio Projects VTB-1, etc. Just don't expect a night-and-day difference, and don't be upset if it makes your tone soun dirty or if you lose some clarity / resolution. It should be looked at as an effect.
