I've went to a Chevy Dealer to get a replacement tail light for my Malibu and they quoted me 100 and some dollars w/o the isntallation. I then went to Ebay and bought one for 15 dollars. Just depends where you buy it. Ebay will be your best bet.
Go here for more info http://www.volksbloggin.com/2006/03/11/how-to-replacing-a-tail-light-bulb/ Don't worry for the model they aply to the Jettas too. Go here for more info http://www.volksbloggin.com/2006/03/11/how-to-replacing-a-tail-light-bulb/ Don't worry for the model they aply to the Jettas too.
The entire assembly costs 100$. The tail light bulb will cost about 8 dollars and it's a very simple matter to do it yourself. A mechanic would probably charge around 30$ for the job plus the cost of the part. Replacing the bulb is a relatively simple procedure which you can do yourself. If you had a mechanic do it they'd charge around 20 or 30 dollars, but you wouldn't need their help if you took your time and did your homework.
I'm not sure how to seal your leaking tail light...but I had the same problem, even after replacing the original tail lights years later, I ended up with the same problem with the new tail light assemblies. Anyway -- since I wasn't able to seal the tail light assembly, I took the tail light off the car and in the top of the tail light assembly above the area where the water was leaking in I drilled a small hole (that you couldn't see when the light was reinstalled) and then a slightly bigger hole on the bottom of the light where the water collects. The air flowing through the top hole forced the water out of the bottom hole. Start with real small holes and increase a little at a time if needed. I hope this will satisfy you problem.