ok another way to look at this..what is wrong with say for example a 96 grand am se, gt 2 door 4 door whatever you want.... I had one of those for a comuter/bad weather car back when I had one of my Camaros. had the 3.1V6 in it and it was a small car so it was decently quick. AVERAGED 28-30 mpg with that thing and got over 36mpg on highway trips.
Cost me $2-3k for that car and never had a problem with it..was MINT.. and it rode real nice too.
Think of how much money you could be saving if you bought yourself a $5k or less car that gets great mileage. Yes you dont have warranty, but really if you find a car that was taken care of (GM preferably) you shoul dhave no problems other than maintenance costs and a few repairs here and there.. even if you had to spend several thousand dollars on repairs over a few years you are still WAY WAY ahead vs. buying a $15k car. Also you buy a $15k car with some remaining warranty.. you drive it for 2-3 years it will b e worth half that with no warranty..where as you buy a car that is already fully depreciated it will still be worth most of what you paid in a few years.
If your under your parents wing and want to get out, don't start by doing it in debt, especially if your arguement is that you want to save money.
I think the question you need to ask yourself is.. do I want to buy a cobalt to save money, or do I really just want it because I want a different car, because the arguement to buy it to save money doesnt hold together, there are way better ways to save way more money.
Again just something to think about.