if you do it yourself you should beable to do it fairly cheaply.
not sure exactly how much a box would cost, my guess... $300-400. But keep an eye out for wrecks on ebay etc. Plus factor in 5lt tranny oil (can be expensive), and maybe the seals too (say $50). So I'd budget $500.
However i'm sure lots of people are thinking, swap to manual. It would be a bit bigger of a job, and may take more time, $, and cause more frustration... I wouldnt bother.
I'd say its worth swapping, but you might be able to find a KA pintara at a reasonable price. Just swap over your kit and windows etc.