There are a few different kinds of shift kits. I’m speaking from my experience with F-bodies, but most of this stuff should carry over.
There are 2 different kinds of shift kits that I’ve seen:
-1 is a simple line pressure booster that can be had for about $45-50. This runs a switch into your interior that you can change to set how firm you want the shifts to be. These are pretty effective for truck/towing usage. These up line pressure.
-2 is an actual shift kit, and it has a bunch of springs and different stuff that you actually have to install inside of the transmission. These extend the life of the transmission under heavy use and can make the shifts pretty brutal. if you’re pumping out a bunch of horsepower you’re going to need to upgrade the transmission itself still, but this helps improve the shifts quite a bit for some serious 1-2 shifts.
http://rides.webshots.com/album/228146776wFWLqX
Neither one of these cost $500.