Ok, I need some marketing help. I have registered a domain name for a specific niche market and want to create

a directory to include companies that offer a particular service or product. I am wondering how to approach these vendors to place a description of there product/service on my site with a direct link to there site. Would I offer this free of charge, just to get traffic coming to my site, could I charge a low annual or monthly listing fee, and what would be a practical amount to ask for? If anyone has any thoughts and/or experience regarding something like this, please let me know. The idea is to get traffic coming to my site, but I don't want to just give out free advertising...please help!