the price doesn't change, so you can add all the nights cost and if you want 5, then subtract the 5th one as it's the 5th night free benefit
"After the 5th Night Free benefit is applied, eligible members will be charged 0 Points for the 5th night, 10th night, 15th night, and 20th night of the stay, as applicable. All other nights of the stay will be charged at the applicable full Standard Room Reward price."

or am I missing anything?