You are in the process of purchasing a new home that will cost you $527,500. The developer is offering you either a $12,500 rebate (applied toward the purchase price) or 6.15% financing for 30 years (with payments made at the end of the month). You have been pre-approved for a loan through your local credit union at an interest rate of 6.5% for 30 years.

You can either take the $12,500 rebate and finance your purchase through your credit union or you can forgo the $12,500 rebate and finance your new car through the developer. The rates are annual rates. 

What is the lowest effective borrowing cost you can get assuming you will remain in the house for 7 years?