I have no idea whether my system will prove to be accurate, but it's a start:
Number of users * avg page views a day = total page views/1000 = inventory. Then do a % of your inventory of sold as CPM * a CPM rate. Next do a % of your inventory sold as CPC * a CPM rate. Then can ad affiliate marketing % and so on.
Build all that as a spreadsheet and you can play with all the numbers with all the numbers as you do research in your space as mentioned on previous comments.
Number of users * avg page views a day = total page views/1000 = inventory. Then do a % of your inventory of sold as CPM * a CPM rate. Next do a % of your inventory sold as CPC * a CPM rate. Then can ad affiliate marketing % and so on.
Build all that as a spreadsheet and you can play with all the numbers with all the numbers as you do research in your space as mentioned on previous comments.