It's been touched on elsewhere in the thread, but as someone who has been looking for 6 months I think it's worth pointing out that there is at least a little bit of "thin market" dynamics happening in the US for the big rigs. Low volume, low supply, high prices.
For example, if you know exactly what you want and only GXV makes it, there are probably 0 on the used market. When it finally does come up for sale, you're not the only bidder. The other option is to buy new, and that's where the $500k+ price tags can be found. If there were simply more of these vehicles out there, I think both the new and used prices would settle down quite a bit.
It doesn't look like this in Europe. More trucks are for sale, and there is a relatively active used market (see Expedition Truck Brokers). They don't list the asking prices often, but I've seen some really reasonable asks on ETB for really full featured trucks. Of course, for many reasons it's not worthwhile to import them to the US and so the market is isolated to Europe.