So are you trying to calculate their possible ROI?
Roi is simply profit divided by expenses such initial investment and both fixed and variable expenses.
To calculate a buyer of lead's ROI i would aim to average out the cost of leads package versus their average product sold to these leads. And simply follow that formula. You can use this roi % for marketing purposes but without other data you shouldnt use it as a tool for their measurement.
With that said you can also assume the average out the a firm's hourly caller salary, assume the average time to close/how many leads per hour and how many sales per hour from those leads. The cost per caller plus leads used that hour can give you a good Hourly or daily RoI for leads sold to a client.