To use, simply enable the Dynamic Pricing extension when loading your campaign.

Many things can affect the price of an item: availability, quality, condition, legality, just to name a few. The customer's reputation may garner better (or worse) prices.  Character may try Persuasion or Intimidation (and may succeed or fail). The merchant may be greedy or in a foul mood that day.  And larger considerations may have a significant impact. Is the city under siege? Have bandits been disrupting trade routes?

Select the item (or just enter a base price), configure the relevant factors, and use the adjusted price.