I am not sure, I have seen much rarer toys go for a lot less and it only takes one guy with a massive max bid to drive the price up insanely. I think that there are more than a few people out there who are basically at the same stage of completing G1 as I am and everytime any of these toys show they dump ludicrous amounts of money at it. As Fatbot mentioned, this wasn't the case just a couple of years ago.
There are also other strange oddities like why are Autobot Targetmasters relatively cheap when compared to their Decepticon counterparts? They seem to be as common as the other aside from the '86' Decepticon targetmasters.