I think synthia's view of expats is overly simplified (to put it nicely). The range of benefits expats get from their companies varies wildly. I know of quite a few assignees living here (myself included) who would never have been able to come if the package included accomodations in Roppongi. Sure, there are some execs who come over with padded expense accounts but they are in the minority. It is far more common for a company to place the assignee in accomodations which are roughly the equivalent of what they were accustomed to in their home country which, for most of non-exec working grunts, means a reasonably sized concrete apartment not too far from their workplace. And, of course, there are quite a few expats who come on their own without company support. I doubt all of them teach English -- although I suspect that's the majority.

As far as what to negotiate... if your office is in or near Tokyo, try to get at least $3K per month for accomodations. That will *not* put you in Roppongi but it *will* get you a nice security apartment large enough that you don't feel like you're living in a closet with air conditioning. If you're lucky enough to work outside of the Tokyo area, you can get by with a lot less (like $1500 or so for a nice 2-br place).

Depending on the company and how badly they want you over here, you could try for:
a) cost-of-living allowance
b) company-paid Japanese lessons
c) company-paid tax preparation service (you will have to file in Japan too)

Make sure the assignment includes repatriation in the event you're canned, too.

BTW, I'm currently living in a 3-BR security building with plenty of closets, a large shower room, and a fully equipped kitchen in a nice neighborhood only 10 minutes from Shinjuku for about $2K/month. If I were living alone, I could get by with a 1-BR in the same area for less than $1K. Unless you're planning on not working at all, you do *not* have to live like Mr. Baseball in Japan.