I personally look at the price of a dirt cheap piece of Korean somethingorother, at let's say £180 / month, and the reduced fuel costs, a modern ecnomicial 4-pot vs my V6.
If I spend £150 a month on my Omega, I reckon I'm paying about the same as I would in the above scenario. Yes, 'trouble-free' [we hope] motoring on a new car, but what I currently have is a car with electric everything, comfy, spacious, a cavern of a boot, takes all my tools and still room for a small family to live in, bum warmers, and a lovely noise when I push the right pedal.
That's my 'theoretical' budget. I don't, practically, spend anywhere near that per month, of course; but I use it as a figure to work to, if that helps