The Consumer Financial Protection Bureau has this on-line interactive tool for estimating what a consumer should pay for a home mortgage. Plug in loan type, amount, length, down payment, FICO credit score, location, etc., and the tool spits out the approximate market interest rate in the relevant location and the total cost of credit. The CFPB says it is basing the tool's answers on "data [that] comes from actual lenders [which] is updated every day." (emphasis added)
And go here for the agency's explanation of "the 7 factors that determine your mortgage interest rate": credit score, home location, home price and and loan amount, down payment, loan term, interest rate type, and loan type.