I have not taken the mono plunge yet, but many speak well of the Miyajima Premium BE mono. There is a less expensive version of the same cartridge without the "BE" designation, just called the "Premium". So far as I can tell, the only difference between the two is that the BE has a black ebony body, whereas the Premium uses some other kind of ebony. This is based on info at the Miyajima website; the US distributor once wrote that there is a functional difference as well, FWIW. I don't know what to think.
There is an AT mono cartridge always available on eBay for just under $500. Might be a place to start.