Hey Paul,
Well, first an observation, then a suggestion.
Since IMHO, the essence and main focus of rap recordings is the story being told, and the story being told is mostly rhythmic spoken word surrounded with cool, quirky rhythmic sounds and music, I'd be looking at mics that tend to emphasize the frequencies that make the spoken word sit out front of the sounds and music accompanying the spoken word.
Sounds simple, but things that appear simple aren't always easy. As an example... what type of voice do you have? Is it low and deep, high and bright, mid-rangy, etc.?
Also, how would you describe the tonal character of the sounds and music that accompany the voice? Do these accompanying sounds and music tend to occupy the same frequency range as the voice? These are just a few things to consider.
In the studio, you're not usually faced with feedback or bleed issues compared to doing live performances, so you can usually choose from a greater number of mics.
There are lots of dynamic and condenser mics that have "tailored" midrange and upper mids to help a vocal "cut" through other instruments. The Shure SM-57 and SM-58 (both dynamic mics, less than $150 each) fall into this catagory. The Studio Projects C1 (LD condenser, less than $300) might also fall into this catagory. Lots of other mics do too.
The preamp and other components of the recording chain will influence how the vocal sits in a mix, including the EQ and the "placement" of the tracks in a mix itself. If you have a friend or local music store that will let you try a bunch of mics before you buy, you can compare and decide which mic sounds the best with your voice and style.
What works well for one artist may not work for another. Also, a "high price" doesn't always mean "better sounding". Let your ears be your guide.
Good luck!
