I'm working on a spreadsheet that I'll send to customers who are interested in switching to Tivo from their current crap cable boxes. Much of the dollar amounts are known, like the current cost of a Tivo Bolt, Tivo Mini, yearly Tivo subscription, and even the monthly cost of a cable card.

What I don't know is how much the customer is currently spending for their cable boxes, and how many they have. I've made a spreadsheet that has only two boxes for the customers to enter that information. It then decides how many Tivo Minis they'll need, and gives them a total for the initial cost as well as the fixed yearly cost of Tivo Service and a CableCard (currently $210 on Fios, for example).

The formula I'm trying to create is sort of a "break even" point formula. I want to see how long (in months) it will take for the customer to start saving money on this investment. I need to factor in the initial investment plus the ongoing costs.

Does anyone know how I make a formula that can do this?