When you say "active leads" what exactly are you referring to? Anyone who is not unsubscribed, email invalid or marketing suspended? or anyone without an open opp?
Depending on your criteria, you could probably do this in RCE. It would be easiest if you do a report on the leads in each stage of your revenue cycle model. You can customize the month you are looking at, and view inflow, outflow and balance for the month of each stage.