I did some number crunching and came up with a formula that I think might work. It's complicated but I don't think there is a simple formula that works.
maximum purchase price of a truck = 6000 + net worth / 25 + (salary/5) x (2-(6 x monthly payments) / (monthly salary))
So a person with a net worth of 250,000 making 100,000 per year with monthly payments of $2000 should not spend anymore than 6000 + 10,000 + (100000/5) x (2 - (6 x 2000) / 8333) = $27,200on a truck.
Assuming he puts 20% down and amortizes his loan over 4 years his payments would be $652/month and his total monthly payments would grow to $2482 or 30% of his monthly salary ..... I would say he is about maxed out.
If he has no debt and he really wants to spend more this formula would allow him to spend up to $56k on a truck. If he finances 90% of the money he could expect his payments to be
around $1115/month
