If the servers are 20% more efficient the amount of dollar sales they should be doing more each month are a total of $660 ($3,300x20%). If they do $660 more per month divide that amount by the redesign amount of $25,000 and you will get the number of months it takes to recover the cost. If you are wanting to recover the cost based on a 20% increase it will take 38 months to fully recover the cost.
