Calculating you ideal payment:

As a rule of thumb, you need to take your monthly income and divide it by two.  
Then, subtract your mortgage, car payments and any other expenses and what's left over is what you can spend a month on a vehicle.  

If you make $4000 per month and have credit card payments of $300 per month and a mortgage payment of $1000.  You would figure out your monthly payment as follows:

$4000 divide by 2=$2000 minus $1000 (mortgage)=$1000 minus $300 (credit card)=$700.

You can spend up to $700 per month on a vehicle payment.