It shouldn't change as far as I'm aware (I paid $6 a year ago, and I have gained access to every new update and every audio log by just accessing the Download tab next to the "You have paid X amount of monies" notice at the top of the game page).
I assume that if a higher price item ever gets added, you could unlock it by just paying the difference between your previous purchase price and the price of the new tier, but as I said, the prices on this game haven't really changed in over a year.