Help with monthly revenue

All -

I have a question:

As a company, we have two types of revenue -- monthly and one time. In order to accurately predict our sales pipeline, we need to track and calculate both based on the opportunity probability. However, there doesn't seem to be any way to replicate / create a field like "expected revenue" to make it based on other, user-created fields.

Is there a simple way to do this? The current "amount" and "expected revenue" don't really work for us.

Thanks,
-Ben