I have an infiniti Q45 1991, my transmission is shifting, how much would it cost to fix?

If Manuel the synchronizing gear needs to be changed and they dont cost much If Automatic the belt (if cvt) or the clutches (if planetary) r weared out

The cost could range from $100 for fluid change to more than $2000 for new tranny. Here is a simple test to get an idea on the internal condition of tranny. Take a clean white cloth and wipe down the tranny dipstick. You should see the pinkish red and some brownish black residue. If there is an excessive amount of residue you have considerable internal wear on your tranny. This could mean plugged filter.