I don't think that it is due to the new gTLDs. The lack of money and the change in domain parking revenues probably have more to do with the decreases. There are some trends that are similar to those seen in 2002/2003 that are appearing in domain statistics at the moment and there is a lot of consolidation (registrants concentrating on .com and .cctld and dropping other gtld domains). I'm not tracking prices though. I only track domain statistics and trends.
Some in the industry believe that the new gTLD entrants, by themselves, will create such confusion as to only further consolidate and enhance the utility and value of the initial generic / gTLD heavyweights.
Given the miserable lack of success of the many aftercomers I'm not getting too excited about their arrival . . but some fools will rush in, chatter a lot . . pump and attempt to dump . . and slowly . . quietly . . harvest their regrets . . while the new gTLD operators attempt to discern their exit points and strategies . . such as "Do we sell our basket of gTLDs to GoDaddy in hopes that they believe they can turn a profit?".
Look for the move . . a last dying gasp of the new wave . . brought to you and I by . .