How to Calculate cost per Lead?

@Mounika Geereddy 

In simplest way here is the Salesforce context to calculate the cost per lead 

1. Leads being captured, and when qualified, converted into contacts. When involved in a marketing campaign, Lead and Contacts are associated with the Salesforce campaign (becoming a campaign member).
2. The campaign cost is stored in a field on the Salesforce campaign. Typically, this requires users to input the amount manually.

The challenge here is the channels from which the leads are being captured. In your case if you are using google add then cost will really depend on the bidding of keywords. So just entering the cost manually on campaign will not might make sense. 

Now how this can be achieved. 

• Store campaign cost per campaign member on the campaign member object.
• On Lead create a roll-up summary field that aggregates the total number of campaign costs per lead (or averages them depending on which metric you want) for the total number of campaigns for which that particular lead is a member 

Formula - Cost Per Lead is it based on Actual Campaign Cost / Number of Campaign Member.

Then create a report considering above pointers. 

I hope this helps.