3.1 Litre V6 engine, 189k miles on it and apparently the Transmission Slips and Gets Worse as the Car Warms up. the car is automatic if that matters.

could be a couple of problems but it should cost any where from 200. to 1000. depending on how serious the problem is

To rebuild.. $1500 to $2500. Best guess would be about $2200. To rebuild and most likely that is what needs to be done l have no idea where 200. to 1000 comes from in other answer. Maybe they are thinking about a used transmission, then that may be possible.

Ok roxy,
you can go used route and find one as cheap as 500 to 1500 depending mileageage and the junkyard.most trans shop will cost from 1200 to 3000 if they try to over charge you. imo try to get a quote from at least 3 different shops.
make sure they try no to over charge you for the small things like trans fluid and odd thing that really do not pertainine to the repair.
ps trans fluid should not cost 5$ quart.

l got the tranny rebuilt in my 93 Grand Prix with the 3.1L MPF l engine it cost me 1,900 and l was having the same problem as you. The car now runs way better and so much faster. They will say that you need a rebuild because its a wast to work on a tranny with that kind of mileage the clutch packs gaskets seals shift solenoids pressure regulators ex are all worn from age and mileage. The 3100 SF l V6 in your car will run for 200K plus if its bean properly taken care of.