Lowest of SO price Vs PGI price

Hi All

We have a scenario where price should be detemined based on the Quotation validity.

We have 3 variables- Quotation Validity, Price on the Quote creation date and Price on the PGI date.

With in Quote validity period we have to bill the lowest of Price of the Quote creation date Vs Price on the PGI date to the customer.

Once Quote is expired,Price on the PGI date is the price we want to bill the customer.

We are currently copying the pricing date from Quote to Sales order but want to implement the above logic during billing or we are also flexible with implementing the logic in SO and then copy to billing since in our case Sales Order, Delivery and billing is created on the same day.

thanks