Is converting from a manual to auto expensive

I have an opportunity to buy a 92 Fox Body GT that has been grannie driven and in pristine condition for next to nothing. I'm looking to build a Fox Body race car and want to convert it over to an auto tranny (i.e.TH400, powerglide, etc). I was curious how expensive something like that would be. Any info would be appreciated. Thanks.

Get a good built C4 for ~900 bucks, flexplate can come from a JY...Wouldn't be expensive at all.