.5% to AGI for the monthly cost. So, if you make 200k per year (and on the interwebs everybody makes at least that) you could pay up to 1k per month. If you make 50k, then $250 is the limit.
It's not the monthly payment that matters, it's how much it costs overall/year. If you buy a new Range Rover for $85k and sell it for 45k three years later, it cost you over $1k per month to own. The payment should be irrelevant if you're doing it right.