A new tip tranny is like $5k+ I have even heard of prices in the $8k range.
You could probably get ahold of a lower mileage tip in better condition for $1-3k.
If it drove/shiftedOK before you took it in Id just have them flush the fluid and wait till it dies. If you took it to the mechanic because of shifting problems then thats another story.. Not sure what the story is on why you took it in in the first place?
Metal filings in the fluid is pretty normal, to a point.

 

Well, if I went with new, I'd say screw it, and just buy myself a different car. New tranny is $4200 from the dealer. Used also known as "salvage" tranny's are in the $1700 range, I believe that's what I'm going to have ordered today. Being in a small town doesn't help much with foreign car labor either. Since they are the only Import place I trust to work on it, it's going to be about a 6 hour install (I call B.S.) at $90/hr. So I'm looking at a pretty penny, but still cheaper than a different car.