Lower the price and the ad will go to the top (ad price history).
When renewing, the user will edit the ad and lower the price (minimum 1 €), the ad will go to the top (option to put the old price scratched). If it subsequently increases the price, it will be taken off the top. Visually, put a green arrow down (improved the price) or a red arrow up (price is rising) if the price goes down or up (ad price history). Create a module with the best descents or with the tendency of the price to rise (for example in the auctions).