Question : Calculate revenue (CPM) from page impressions.

Lets say I get 5,00,000 page views. Fast click pays 720x90 leaderboard banner == $2 CPM= $1.24 eCPM.

How much does it mean in revenue? A typical page has 2 -3 banners. Help me understand.

Answer : Calculate revenue (CPM) from page impressions.

Honestly I can't say because it varies too much - from advertiser to advertiser, from website to website and absolutely the combination of the two.  Making matters worse is the changing conditions - meaning what was the norm 6 months ago doesn't mean it is going to be the norm now.  

The only way to tell?  Trial and error.  Try one offer - find out if it works for you but at the same time always be on the lookout for others.  If one works really well... stop looking.  (but always be ready to start again, because even great advertisering firms fail sometimes - or a change in policy could dramatically drop your revenue).  

When you try a new site, don't give them all of your space, use a script that will rotate the different services and give them a portion of it... you will see what works and what doesn't.  Give the higher percentage to what is working for you.  (Another great thing about rotating different advertising programs is that many only give you credit for each IP per 24 hours... so rotating to a second gives you credit for two different programs with the same user if they go to a second page.  I used cookies for this.  The first banner program displayed was my highest average CPM, the second banner was the second highest...  At one time I was pushing 110-150,000 pages daily to 30,000 uniques a day so I had 4-5 different programs running at once.

The short answer - experiment...  (=

Hope this helps,
Matt