Prices can indeed change. There are (at least) three possible scenarios:
1) Pricing was incidentally set incorrectly, or reviewer's decision about pricing was not the most adequate; author contacts support, they consider the pricing and author's request and if author's right, price gets changed.
2) Author updates his item. He/she adds more value to it and their item becomes more complex, so its price goes up. (or if they remove something from their item, price goes down )
3) Envato decides it's time to change (hopefully, to raise ) prices because of the trends in the market in global, so they change prices of all items of a specific category or on whole marketplace.
As you can see, pricing can change, but it happens on a very rare occasion. Unless you have thousands of items you'd like to advertise, I suggest you go for it and put the price on your banners.