Here is a one of the ways to calculate PPC profit:-
Sales Leads per day * Percent of conversion * 20 work days in a month = Number of conversions
$ in Profits per conversion * Number of conversions = PPC Budget Maximum
Total $ Profit from PPC - Total PPC Budget = PPC Profit
In the above formula, some of the items can be substitued with assumptions like sales leads per day and conversion rate specific to the industry.