The principal & interest payment is based on the total loan amount, and the interest rate you are being offered. For the payment example let's use 3.25% on a 30-year term (I am not quoting a rate, it is just a rate being used for the example). The principal & interest payment would be $3,204.92/mo.
Next you have the annual mortgage insurance (MI) that FHA charges. The rate of the annual MI will depend on the down payment percentage, loan term & loan amount. The annual MI is only paid on the base loan amount (not total loan amount). For a 3.5% down payment on a 30-year term on a loan amount above $625,500 it is 1.5% per year. On the base loan amount of $723,750 that would be $904.69/mo.
You also have homeowners insurance, which you should get a quote for, and property taxes which are normally estimated at 1.25% of the sales price here in California. FHA loans require an escrow/impound account, so those items will be a part of your monthly payment. If you estimate $1,200/year for insurance that would be $100/mo & 1.25% of the sales price would be $781.25/mo in property taxes. All of that totals up to $4,990.96/mo.
Current FHA MI rates: http://portal.hud.gov/hudportal/documents/huddoc?id=12-04ml.pdf
Shane Milne | Lending in all 50 states | NMLS #81195