Need a pricing algorithm to bypass updated Price list during a Contract usage subscription term

We have an active Contract Usage Subscription with a product whose price is say 'X' during the start. Somewhere along the way , the price of this product changes in the OM to 'Y' and this in turn has changed the price in the ongoing Subscription. The requirement/ understanding of a Usage Subscription is we would like for the Product to continue the same price until the end of the Subscription term. the Pr

Requesting for assistance with customizing the pricing algorithm to accomplish this scenario.