[capnfabs]

I'm wondering if it's got something to do with saturation on the demand side. I'm going to take a (roughly) economic angle...

When AdWords first launched, it wasn't (necessarily) a standard for online advertising, and especially for advertising generally. As more businesses consider it to be part of basic strategy, the cost of your bid goes up to remain profitable.

The previously obtained performance is the anomaly - it's a first-mover advantage on a new technology. You get more bang for your buck because you're making use of an asymmetry in the market-place.

The phenomenon that's occurring here is commoditisation - Adwords is well-known and well-used; it's a standard. The competitive advantage that comes from its use has finished.