I tried doing the two side windows on my miata and was a pain. Now if the tint is already cut to fit then you may be in luck and it could be easier. My opinon though is to pay an expert, this way they have to fix the mistakes.
The way I see it: if you have an expensive or nice car, and want to keep it looking nice, pay someone to do it. That is, unless you work at a tint shop, the tint shop will let you do your own car after hours provided you pay for the materials, and you have at least one tint shop buddy helping you
If you have an older car that is not so nice, a beater, or something you don't care about, and can get a friend who will coordinate with you, you can do it yourself.
I've helped a friend tint a car in the past. It was both his and my first time doing a tint job, and it actually turned out ok. The edges of the windows weren't perfectly even, and there was one ragged cut out over one of the door locks (the car had those stick-up cylindrical door locks that partially block the window) but in the end it wasn't too bad. But, this was an old Lemans (the rebadged Korean kind made in the late 80s, not the original muscle/family car Lemans that could be a GTO) so he wasn't concerned about looks.
If you pay someone to do it:
1) You'll get a warranty, so if after a couple weeks it starts peeling, or after a year it starts bubbling or fading, you can take it back and someone else will redo it. Removing tint from a car can be a bear.
2) If they screw up putting on a sheet, they throw it away and get a new sheet. No running back to the store because you suddenly ran out of tint because you had to scrap a few pieces.
3) Chances are, it will be applied more securely, and aligned better, than you can get it done yourself. Additionally, it should last longer.
4) From what I've seen, you cannot buy the higher quality tint in do-it-yourself kits--only the lower end stuff.
For doing it yourself, you need 2 people. The two people have to coordinate well to hold the sheet of tint out so it doesn't start sticking to itself and wrinkling/tearing. You need to very liberally spray the soap-water on the window, and keep it wet at all times. You will be spraying lots of soapy water. The 2 people need to coordinate to very quickly get the tint sheet on the wet window, and slide it carefully to align it. Then it is spraying the exposed side of the tint sheet, once again keeping it very wet, and squeegying it down with the rubber squeegy tool.
I can't remember how long it took us to do his Lemans, but it was at least an hour.
For a 30k car, I'd pay someone. Depending on the type of tint, you can pay $100 (cheaper, basic dyed tint) to ~$300 (ceramic tint). Tint prices do vary though--if you live in LA it is probably a bit more than what I listed above.
My friend conned me into helpin him tint the wondows on his C4 Vette. It was a nightmare, took us about 3 hours and it came out like crap. I had it done professionaly on my Goat. Was well worth paying to get it done. Especially now that most places use a machine to cut the tint to the right size of your windows.
I'm having mine done tomorrow...I've had 2 cars done by the same guy (accord coupe and accord sedan) both were great and if there were any bubbles or anything he would relpace for free. If any guys in MA need a tint guy let me know...no hablo englais though.