Which would be cheaper

I can get a used th400 for near nothing, so how much would a good rebuild kit for it be.

or would it be cheaper to buy one already built.

probably around $200 in parts if you do it yourself

~$225-250 for a full kit. But you may need some other parts, there is no guarantee that some of the hard parts aren't broken or worn. I just did a Powerglide and ended up needing a front pump and a planetary. Luckily, my buddy had a spare planetary but the parts still ended up >$500.

Rich

if you can find a built one for about $600 you'd come out better than rebuilding one. rebuilds are hit and miss.