Why would sony bother to buy exclusive rights last gen? Ps/Ps2 had the largest install base, any company with the right mind would make their game for PS/PS2 unless they were targeting a specific audience. This gen has more multiplat because Xbox and PS3 have similar core architecture, which allow companies to create engines that have "Make Xbox game" and "Make Ps3 game" functions.
Building Multiplat games is so easy that exclusive rights price tags are heavy, I seriously doubt Microsoft or Sony would buy them often these days. Exclusives this gen happens when the developer is owned by Microsoft or Sony or if the developers have something to prove (Gears, UC2, MGS4).