Price of dwelling unit for sale

We are trying to determine the price of affordable dwelling unit. With regards to new for sale dwelling unit, we can calculate maximum monthly PITI according to the Reference Guide. But I don't know how much it'll be allowed for affordable dwelling unit price. Many people in Japan make loans for 30 or 35 years. So, is it possible we calculate affordable dwelling unit price as below? maximum monthly PITI × 12 month × 30 (or 35) years. (e.g. PITI=$1,000 → affordable dwelling unit price=$1,000×12×30=$360,000) Of course, I know that interest rates etc. must be considered. May I decide the number of years of mortgage arbitrarily?