purchase price maximpurchase price maximum is determined based on a percentage the financial institution believes is a wise amount; this amount is often around 30% of gross income, depending on other debt and tif the income level is $50,000 per year, the amount of yearly mortgage payments should not exceed 30% of that. Therefore, 30% of $50,000 would be $15,000 spent per year on mortgage payments. Since there are 12 months in the year, this would allow a monthly payment of no more than $1250 per month.um is determined based on a percentage the financial institution believes is a wise amount; this amount is often around 30% of gross income, depending on other debt and the credit history. For example, if the income level is $50,000 per year, the amount of yearly mortgage payments should not exceed 30% of that. Therefore, 30% of $50,000 would be $15,000 spent per year on mortgage payments. Since there are 12 months in the year, this would allow a monthly payment of no more than $1250 per month.