I swear - I hardly ever browse this forum anymore, but every single time I do, there's a "rap mic" thread on the first page.
. . . . . every single time
Now, if we could just combine essence of the "rap mic" and "studio projects mics" threads in one big massive worthless-waste-of-time thread, we'd have something.
Sorry, truth_lyricist, I really didn't mean to hijack your thread or to be rude - but this topic has been done to death around here.
My advice to you would be to find a dealer in your area that carries several mic's you are interested in. Go in there and find a nice salesperson that you get along with. Arrange a time between you, your rap vocalist, and the salesguy to meet in the store and audition mic's. Any real dealer will allow you to do this. Bring in your backing tracks on a CD or something, set up a signal chain as close as possible to your own that you can, and audition all the mic's you can in your price range. Pick the one that sounds the best to you. If you do this, you'll get the mic that's best suited to your vocalist, you'll develop a good relationship with a local store and sales staff, you'll learn about the tonal differences in different types/brands of mics, and you'll have some fun in the process. You also might find something that's perfect for way less than $700. More money doesn't always mean better for what you are trying to do.
Good luck!