I work for an Autolending Department and we are trying to further develop a simplistic method for estimating what we call a true buy rate. We are an indirect auto group and publish our rates to our dealers. Our dealers have a choice of increasing that rate with a max spread of 2% in which they keep 75% of the Markup and we keep 25%. In the case they use the rate we publish no mark up we pay them a flat fee. We have a simple formula the estimates (not perfectly) what the buy rate is when they mark it up (we increase our buy rate by the 25% the bank has the ability to make in finance charges) or when we pay a flat fee (we decrease the rate by the amount we pay the dealer).

Below is the formula we are using to estimate what we consider the True Rate.

If dealer chooses to use spread.
[Buy_Rate]+(([Amount earned from Markup to Bank]/[Finance Charge])*[Buy_Rate])

If dealer chooses to use rate published (no markup)
[Buy_Rate]-(([Flat Fee Paid to Dealer]/[Finance Charge])*[Buy_Rate])

If anyone has a more accurate method please point me in the right direction.