2000 seems to be the average for a rebuilt transmission. If you go used you might have to rebuild that one too. I go for a rebuild IF its a reputable shop, with a good warranty. Also i'd ask if they are going to use a 'Master Rebuild Kit' and will they redo the servo bores, which is kinda of required, also do they have plans on replacing the Valve body (optional)? If they do all of the above minus the VB as they did with mine, it will be better than new. I paid $1600 for mine to be rebuilt 10k ago.
another idea is look for something called 'crate' transmission which is already rebuilt, it would be close to DIY, if you have the right tools.