First, I'm not associated with any of the folks linked to below, but I've already done the research, so I'm just sharing that bit.
For somewhere between $250 & $700 you can end up with a mic that compares well to classics that start at 4X those figures, if you can even find anyone willing to part with them.

Here are some links to consider. Some are ready-made mics you can order, some are customization services, where you send your stock mic, and they send back something closer to a high end "boutique" mic. Spend some time poking around the sites, I'm just including one or two links per vendor. Follow up on Google, for opinions from folks with hands-on experience:

http://www.busmanaudio.com/bal2.html

http://www.oktavamodshop.com/product_info.php?cPath=1_46&products_id=126

http://www.aamicrophones.com/fet_mics/cm_47.htm

http://jjaudiomic.com/Mods-for-Fet-Microphones.php

http://www.oktavamodshop.com/product_info.php?products_id=137



Also, don't forget ribbon mics as an option for studio use:

http://www.shinybox.com/ribbons.php

http://www.oktavamodshop.com/product_info.php?cPath=1_24&products_id=112



And finally, a detailed example of how to emulate a mic, based on it's known characteristics, using a similar mic and audio processing finesse:

http://homerecording.com/bbs/showpost.php?p=2597383&postcount=37