I received the following explanation from Jon when I queried the WordAds support site:
"Sure. Thanks for asking. It is probably the most common question we get. There are a few reasons why prices for online advertising rise and fall. One is that there is not a set price for an ad impression in online advertising. It operates just as a stock market with prices rising and falling in real time and with prices for visitors from different countries being dramatically different.
Another reason is that we don't have 100% fill rate for ads. We could but the low paying ads are terrible so we chose not to run them. Fill rate will rise and fall.
A further reason is that we were running high paying ads from Federated Media to your US traffic. Federated has abandoned this business: http://www.adweek.com/news/technology/federated-media-shutters-standard-direct-sales-business-145069
The other partners pay well but not as well as Federated. We are always testing new partners to try to increase the payout. I hope that helps. Jon"