Question on the reference price

I just want to clarify the definition of reference price.

I understand this is the price the product is sold for over the last 30 days period. However, If I run the coupon on this product during these 30 days, say, the original price is $100, after coupon 10%, the price with coupon used is $90. Once after this 30 days period finsihes, what is the new reference price? is that still $100? or it is $90? If it is the latter? does that mean every time seller wants to use the coupon, the final sale price will be lower and lower?