$120/month with my parents full coverage
You insure the car not the driver, and yes they can find out if you put the car for pleasure but you use it as a daily driver. You give them your mileage and if you get into an accident and they see the odometer they will probably deny your claim.
Quit being a cheap ass, if you can't afford the car and everything you need to be able to drive it then don't get it.
All of this info is wrong.
In any case... it's going to be a lot. Even if you are rated as an occasional driver on the car (this can only be done if there are fewer cars than drivers in the household), it's still only going to make a small difference. It does matter if you go through your parents company though, because you'll get other discounts (multi-car, multi-line, ect). Expect anywhere from $200-$300 a month.
BTW, you said that you want a "just enough to be legal" quote. That's a HORRIBLE idea. You can get 15/30/5 in many states. That means if you hit another car, you're only going to have $5k in coverage to repair it. Lets say you hit a Lexus IS300. You're going to either have to pay $20k out of pocket, or be sued. The difference between legal minimum and something like 25/50/25 isn't going to be that much. Especially since you're young and have a good chance at getting into an accident at some point, I'd highly advise you don't go bare-bones on the liability limits.