If you opt to go the used route, there's the Audi 016 5 speed which is marginal at the power levels you are discussing (although some people swear they can handle 450hp with ease). Cost on these vary but are typically under 500 bucks.
Here you have the various flavors of used Audi gearboxes, the 01E, 0A2, 01X etc.
The only gearboxes I'd recommend for 400hp and up are the 0A2 or 01E.
There's the upgraded and re-manufactured 5/6 speed Audi 01E's, which start at the mid 2G's and up.
3K and up is your used Porsche gearboxes in various configurations.
Ken,
Just be aware that contrary to all the other trans mentioned the Audi must be fitted with the input shaft above the driveshaft line since it can't be inverted. Hence you have an undesirably high engine position. For a low priced option the Renault is maybe a better proposition.