They're trying to keep all the Prereleases consistent with other Prereleases in the area. If an event offers a lower price, some would attend that Prerelease over another Prerelease. If one is offering to add prizes for the top players in the event, players will attend that Prerelease over another.
We have done $18 since the change in prize structure and thrown in two boosters into the top eight prize pool per entry in each age group. Another Prerelease was already charging $20 before the prize structure changed, still does charge the same amount, and no extra prizes. We were going to remain at fifteen but we decided to improve the prizes for the top ranked players at the end of the event. So yes, even a difference in price can cause some competition.