Deborah is correct, but there is more to it than just payment. A mortgage payment is tax deductable, so if you are currently paying $1200 per month in rent, you may actually afford a higher mortgage payment and have the same budget impact. There are various rent/buy calculators on the web, they are all based on different assumptions but worth knowing about.
The best way to work all this out is to start with a lender and get pre-approved for a loan. You don't need to have the property you want to buy picked out.
When I do this for my buyers, (I'm a licensed mortgage lender and Realtor) we end up with a range of ideas of how much home you can buy based on payment, loan program, down payment and other expenses. It all has to work together. Find a local mortgage broker and or ask your Realtor for a referral for one and see where you end up.
Good luck home buying.