For starters, with my business model, the monthly expenses depend on the number of customers directly. Every time I get a customer, I automatically deploy a VM. And I am charging them a few more bucks than the VM costs, so my revenue is directly tied to the number of customers also.
Maybe I just need a spreadsheet. But really its pretty simple. I plan to launch something that I can support alone. Within a month or so, I need to have enough profit to hire a guy from oDesk to help with support.
The other part is charging for support, which I plan to make separate from the servers. So you can pay as low as $5 if you want minimal support, up to $500 if you are a business and want to prepay for up to half a day of consulting each month.
So what I want is a spreadsheet that I can change that has a projection chart off to the side that is automatically connected to the spreadsheet.
With template spreadsheets/charts that I can configure.
Maybe use something like Google Spreadsheets or http://stoic.com/formula/index.html