Know-How Exchange : Advertising/PR

Hello,
I am the producer and owner of a 13 episode TV adventure series. It has aired worldwide on National Geographic among others. Up until now I have been working with TV distributors who license the series to a broadcaster and take 25% of the fee. It hasn't proved to be very lucrative. After hearing from some broadcasters in the US say they only barter for airtime, or will accept barter, I'm very curious about how it works. I have many great relationships with corporate sponsors and ad agencies who were involved in the event, 50+. Many would consider placing spots in the series as they were running commercial campaigns during our production i.e. product testing etc. (it is an auto-based adventure series) There are 13 x 30min shows each has 3 commercial breaks. I understand that the broadcasters often take the series for free in exchange for about half the spots or roughly 9 per show... (3 from each of the 3 breaks) Supposing this is accurate, I'm assuming I sell back this time to participating sponsors for slightly less than it would cost them to buy it direct from the broadcaster?.. Should I try to squeeze in a couple :15 sec Billboards "Brought to you By" to make it more appealing? Is there a typical format for creating a buy opportunity like this to present to Ad Agencies? Any and all suggestions will be greatly appreciated.