I wrote this spreadsheet probably a decade ago. I used it for estimating the audience for a site, and calculating the cost/value to include certain demographics or technologies in a site. Feel free to clone and fiddle with it.

https://docs.google.com/spreadsheet/ccc?key=0AhtUWQPTxSwVdG1FMU54X0VCY19iWEJFaFk2Vkd3NFE#gid=0