I'd say you're overdue for a change by about 20k miles, especially if you haven't been doing any of your own maintenance (4qt drain and fill every several thousand miles). $70 sounds pretty good for a complete flush, but they'll probably charge you the same amount whether or not you bring your own fluid, so you're better off not to. Find out if they supply Amsoil ATF as well, and just go that route. Keep in mind they'll charge you more. My local shop charges $90 for a flush, and if you ask for synthetic, they'll add on the difference.
