This is mostly anecdotal, but I do believe that the retro craze, including prices, has peaked and it is gradually declining. I have been trying to downsize a little recently and have several highly collectable, desirable games on ebay at the moment, all at competitive prices and no one seems to be biting. it isn't just me either, people either seem to be waiting for auctions now or someone to post a specific game really low, which brings the price down overall even more. I have noticed this on nearly every game I've been selling, which again, is a personal account, but having sold many games over the past several years, I can't recall it being this hard ever to get a fair price for your games on ebay.
The other part of me that thinks that the retro collecting craze has peaked is the amount of collectors I've seen leave the hobby over the past year and a half, some with massive collections (4000+ games). Being a regular at various retro game stores around town, I have also seen for the first time in years retro games remain on shelves for more than a few days, and in some cases gradually accumulate over time. A few store employees have even backed up my statement about collectors leaving the hobby, stating that they are getting people in on occasion who decided to sell of their collections to the store.
I think we as game collectors assume that far more people are into this hobby than there actually is, and I honestly beleive that not only are people just getting bored with the hobby and wanting to move on, but also many collectors have everything they want, or close to it, and see little desire to buy games they've had now for years.
I think what this all means is that prices will gradually come down on everything. Certain games like Contra or Super Metroid which aren't rare, but highly sought after will probably see the biggest decline in price, while games that are genuinely rare will probably still be insanely pricey, but they will get slightly cheaper. Sadly, prices will never return to their pre-collecting craze levels, but hey, as long as games like Super Mario World are going for around $5 again and a lot of the resellers get out of the hobby, I wont complain much.