If you know your fixed expenses, your discretionary spending, your savings, and your retirement plan you should know pretty well what you can spend.

Fixed expenses are easy to figure out. Hopefully you have goals and benchmarks for savings and retirement. Then it's just a matter of determining whether you're already putting more into savings than you need to meet your goals or whether you can make some cuts on discretionary spending.

The cost of ownership of a car is a little tricky to figure out. It isn't only car payments (if you choose to get one), it also include gas, maintenance, insurance (full coverage if it's financed).