Not to bring politics into this, but with the middle class not getting any better off for a while, who is going to be able to pay these prices now or even in 10 or 15 years? I make good money but even I have a hard time justifying spending four figures for a porcelain sign. These signs are so expensive now I don't see a lot of people having the disposable income. Besides, there aren't nearly as many gear-heads around today as even when I was a teenager back in the 80's. I think a lot of interest in gas & oil comes from love of cars and a lot of kids today don't love cars. Looking at most cars today, can you blame them? They're too distracted with the internet, online gaming and their silly smartphones.
In another 15-20 years all the baby boomers will be in retirement homes and I don't think they'll let you hang your 42" DSP sign in the retirement home. So, they'll be a small number of super-rich guys to buy the rare stuff and prices on the rest will drop due to lack of demand/interest. I don't care because I just like the stuff. It's not an investment to me, though it would be nice not to lose a ton of money on it.
BrianH