Palo Alto-based Focus Ventures has invested $15 million in San Mateo-based ad network Turn, which plans to use the money to increase its New York sales presence and provide what AdWeek calls "a cushion should the Internet economy take a downturn." Lots of people think such an downturn is underway — online ad spending is definitely hurting — and lots of people think ad networks are at the root of the problem . Not Turn CEO Jim Barnett, natch.Ad networks, he told AdWeek , are "a very effective way for publishers to sell a lot of their inventory. They can get $1.50 from their network partners or they can get zero." If it were so simple, Focus's investment would make more sense. A recent Interactive Advertising Bureau and Bain & Co. study of seven publishers showed that the publishers in the group who used ad networks heavily could only sell their premium ad invetonry at an average CPM, or cost-per-thousand impressions, of $10.36. The ones which mostly abstained were able to charge CPMs over $20.