This video will walk you through how to update the price of an inventory building. In short, when you edit an inventory we recalculate baseprice + options equals new retail price.
Used and Aging Building Discounts
Our recommendation is to use an addon for changing the price of used buildings and aging buildings. When these buildings recalculate as described in the video above (base price + options = retail price), an automatic percentage can be taken off of the base price or the total price (depending on addon setup) to keep them priced as desired.