Typically we pay leads by a percentage of the deal depending on the Gross Profit Margin. So if I landed a job that lets say material cost would be $20k and I mark it up for material, labor etc by 18% over actual cost my Profit would be $3600.00. Then I agree top pay 10% of my profit margin as a lead which would come to $360.00.
This was a knock off of a formula back in the day where sales who worked for any company who have leads given to them got say 5% commission and if they brought their own leads to the table made 7% which allowed them to cut happy customers a deal for leads to be 2% of the total cost of the sale which had a great trickle down effect from receptionists and company owners on keeping the leads coming in.
I felt 10% of profit is a good lead reward but obviously you could dictate your percentage higher or lower to entice the lead provider and be fair. I would steer clear of setting a single lead $$ amount in case the lead didn't make you very much profit.
Hope this helps.
If It's Not Broken...Don't Fix It!
Stephen Webb
Norstar, BCM and IP Office Engineer