The rental price is fixed. Renting a sсript at the current price, you always extend the rental (from your personal account, by clicking the extend button), at the same price.
Even if the price changes, the previous conditions will always apply to you.
If you miss 3 or more months of rent payment, the price will be changed to the corresponding one on the website.
(!) I would like to note that discounts do not change the price, they only give a discount to the current price.