Given the popularity of drifting...many rwd boxes these days do seem to carry a premium. Especially the Jap stuff.

I think the Volvo M90 is supposed to be quite strong, maybe something not that popular in the UK ? which should mean cheaper.

http://en.wikipedia.org/wiki/Volvo_M90_transmission
But integral bellhousings found on a lot of boxes these days...can make fitment harder.

The box fitted to RB25's seems to be reasonably strong, think they're pretty big though.

Problem is cheapest box...doesnt mean cheapest fitted and working box.

If you pay a bit extra for a box/clutch that just bolts on directly, it will usually be cheaper and more sensible than one you need to adapt to fit, and use custom clutch parts.