Cost Per Lead (CPL) is a metric used to determine how financially effective a marketing campaign is. CPL notably differs from CPI (Cost Per Impression) and CPM (Cost Per Mille) as with the CPL model advertisers generally only pay for qualified leads, rather than paying per impression.
CPL can be calculated by taking the total amount spent and dividing it by the total number of leads generated. CPL is important to measure as it allows you to get a good idea as to how successful your campaign is. A low CPL is a good sign as it shows your campaign is successful and is yielding good results, while a high CPL would indicate that changes need to be made in order to bring the CPL down.