My motoring bill is maybe £300 a month, it's a big chunk of months income, basically the same as my rent, and i OWN my car.
The car was my dad's. I was planning to buy one (more on that in a sec), found a decently priced 1.6L Fiesta but my mum was worried it'd leave me with little room to move when it came to insurance and keeping it going until I was out of school and out at work. My dad was also looking at getting a newer car at the time as well and it all just fell into place nicely for me, bar my dad's car being an older 1.4L diesel, pulls well at low speeds, gutless above 30. But hey, it ran well then and still runs now. Not expecting it to be one of those crazy long lasting ones (I legit still see a fair few 52 plate Fiestas knocking about) since the electrics are starting to let it down, but I reckon it's still got 2-3 years left in it at least.
Insurance, at first, was money my mum had put away for me as I was going up (which I was originally going to use to buy a car and insure it) After that it was money from work.
Fuel wasn't a particularly big concern. Didn't really do the mileage to require filling it up often (also doesn't have a DPF fitted, so lower milage isn't a huge concern for it, not great, but not as bad as it could be), even when I was working. Hell, even now, just going to work I can stretch £10 of diesel about 2 weeks. Give or take a few days. More recently it's been more like £20 a week since I'd be going down to see my GF, well, ex now. But yeah.
I think, at least in the first few years, you really need to pay your insurance all in one so it isn't draining from your funds every month. It's also cheaper in the long term. If you can get that hit out the way, you should be able to cope until it's time for the dreaded MOT and service, and any moments between where the car may, or may not, have issues.
But yeah, I'd imagine most people do it like that, they have a little bump start to tide them over until they get a job. Can't say I really know what the monthly cost of running my car is. It's just sorta diesel when needed and save for the double whammy in September (MOT, service and insurance)