You don't have to pay anything, but you can use this calculator to determine the end cost for your buyer.
The "Introduction Fee" will be put in Salesconx escrow now, and will be paid to the seller when you have received and verified the contact information you're requesting. As an extra incentive, you can offer a "Success Fee", which will be collected along with the Salesconx Success Fee after you verify the contact information. The Success Fee will be paid to the seller if a deal is concluded within the timeframe you specify above, or otherwise returned to you.