Hello Gianna- You can figure out your potential mortgage payments by going onto the following website and plugging in whatever numbers you'd like to work with. See below for the site, and go to the Calculators page.
If you did buy a property at $480K and put down $100K, your Principal & Interest payment would be-
$2309/ month + taxes + H.O. insurance . No PMI needed as you're putting down the min. 20% DP.
The above would be at a 6.125% interest rate.
FYI- Right now there are even better/ lower rates on FHA mortgages (not just for 1st time homebuyers). And a 5/1 ARM on that property would be around 5.75% today. Let me know if I can be of help. Thanks and good luck,