dude it expensive ... you could just need a service with new fluid/filter and kickdown cable adjustment .. do what I said above then put bottle of nulon auto transmission treatment in.. if its that bad then get a 2nd hand one fitted .. are you sure its transmission ? could be stuffed uni joints or diff ...
Transmission rebuilds will be looking in the vacinity of $750-$1000 depending on where you take it and what you want done. Try a service first though, drain the pan oil, throw some trans treatment in to help soften up the bands and seals... then see how it goes I doubt stuffed unis could cause the car to feel like it's slipping between gears. It's almost certainly a gearbox or torque converter problem.
try adjusting the cable by your throttlebody my last vn calais was slipping badly adjusted this cable then was fine
