I usually do my own. Depending on what seller you buy from on ebay, you can pick up quality computer cut tint pretty cheap $~40-60. I've done about 5 or 6 cars now myself using ebay stuff, and have done a few cars with the WalMart tint in the past. From my experience the WalMart stuff will fade (turn purple) pretty quick, within a year or two. I had the ebay stuff on my 04 GTO for 2 years, and it still looked great when I traded it in. I'm pretty sold on the stuff, especially for the price. If you've got a couple of hours to spare, I think it's definitely worth the $$ savings. If you do decide to go this route, remember, cleanliness is the key!
Lowest quote I could find for my Camaro was $225 for just the side windows.