Actually 400 for the c-6 not including convertor is going to be about right too if you buy most of the trick stuff.Just got through doing mine. The biggest single expense is the "R" code servo which sells online all day long for $150 getting a rebuild kit with regular clutches and band cost me $70 better clutches would have cost more. I paid $50 for a good valve body kit (my is now a manual) I think the cheapest actual valve bodys i have sean were around 150 if you went that route. so by the time you start adding all the incedentals in ATF, brake cleaner and such $400 is actually about right. If you do it yourself be anal about getting it clean and all the metal shavings out of it.
Okay all that said I have not used this guy to build any of mine(i do my own labor) but his parts are good and his prices are good too it is also a fairly informitive site.
http://www.broaderperformance.com/