First separate the two numbers. Gas is gas, and insurance is insurance. One can be thought of as overhead (fixed-cost), and the other is an operating expense.
Your insurance is fixed, in that it costs you whether you drive or not, but the rate will be highly variable depending on your ZIP code, your credit report, the liability rankings of that vehicle, cost of repair parts, the coverage limits and deductibles you choose, and you. Did you just get your license yesterday, or have you been driving for a number of years? Have you carried car insurance in the past? If so, they'll want a good explanation for the gap in coverage, and the only good one in their eyes is if you did not own a car.
As far as liability goes, it's just numbers. If Nissan 350Z owners are involved in more accidents than Toyota Camry owners, they will charge more to insure the 350Z for liability.
For collision repair coverage, if parts are more expensive for one car (say, a new BMW with a lot of aluminum body panels) vs. another (say, a Honda Accord with all steel panels, and plentiful availability of factory, used, and 3rd party parts), the car that costs more to repair will get charged more to insure.
Your ZIP code also matters. In my area, someone who lives in congested downtown or the neighborhoods immediately next to downtown will pay more than someone who lives 5 miles away (and the person living five miles away pays more than the person living in a rural area).
All of the insurance companies look at a combination of factors, and usually come to different conclusions about what to charge you. It absolutely, no ifs, ands, or buts, pays to shop around.
Another fixed cost to plan for is yearly registration. That varies by state, and in many (most?) states, it varies by car, with more expensive, newer cars costing more per year. In those states, the cost
drops each year, until it hits a minimum after about 8-12 years. In my state, the minimum is about $40. The maximum is many hundreds, if not over a grand, but for cars most people can afford, it's like $300-350 when it's brand new, and it goes down every year after that.
Now your gasoline is an operating expense. It varies according to usage. The best way to predict that cost is with a calculator, plugging in cost per gallon, miles per gallon, and miles driven.
You also have to add in scheduled maintenance (fluid and filter changes, "tune-ups," etc.), tires, brakes, probable suspension work, and other repairs as the machine ages and wears.
So, I don't know--start shopping for insurance (or rather, shop for an agent who seems interested in earning your business once you do buy) on some examples of cars you might buy (and maybe a "vanilla" car like a Taurus or Camry for comparison), and grab a calculator and run some fuel costs. Call the dealer service departments to get an idea of expected maintenance costs, and check out Tire Rack and Discount Tire's sites for an idea of what tires will cost for the cars you're considering.
Run all your numbers before you decide to take that plunge. (You might also decide that a non-sports car is much more practical at whatever point in your life you are--because they are less expensive to own.)
Edit: I see you're looking at a Mercedes SLK. I honestly don't think $300 is going to be nearly enough for gas and insurance, let alone everything else, unless you barely ever drive it, and insurance is close to a best case scenario.
I forgot to mention that the major insurer's websites like Progressive.com let you run a quote online (or at least they used to), so run a few of those at the different websites to get a better idea for yourself.
Last edited by Thegonagle; 09-24-2011 at 08:56 PM.