There is a cheaper way.
It'll cost about a dollar- it's for purchasing a resistor at Radio shack. I saw this trick on ls1.com.
The way these "CAGS" (computer aided gear selector) work is there is a solenoid built into the transmission. (same for the reverse lockout) under part throttle consitions it locks out the ability to shift into second gear (when engine is warm) and forces you into 4th.
If you just unplug the wire to the solenoid, you'll make an incomplete circuit and set off trouble codes. This is where the "shift skip" kits and this resistor come into play. The resistor fools the computer into thinking the solenoid is engaged. It helps complete the electrical circuit so your computer is happy yet you have physically disengaged the solenoid so it can not force you into 4th gear.
with the resistor trick you simply unplug the wire to the solenoid, insert the resistor to the wire plug and tie it up so it doesn't fall apart. the solenoid remains unplugged on the transmission and remains off because there is no power source to it anymore.
there is a specific "ohms" resistor required. you can't just use any resistor. unfortuinately, I don't know it off the top of my head.
The exact instructions are on the website, or they were, and it worked just fine on my 1995 Trans Am.
I'll admit when I first did this the reverse lockout solenoid also stopped working for a day, but then resumed working all on its own. I'm assuming the computer had to slightly re-learn or something.
The transmission has 2 solenoids built in. I don't know why the reverse one temporarily stopped working, but like I said, it resumed working after a day.
The shifter light on the dashboard still comes on, and I think it'll do that even if you buy the 30$ kits, too.
I reccomend it because it's a pain to have to go to 4th unless you want to floor it every time you leave a stoplight. that just uses extra gas and puts extra strain on your drivetrain.
Good luck!