It's principles of economics. Supply and Demand... So far Henkei stuff have a limited supply and as such pieces will go for high prices if it is of high demand as well. Inflation in prices for G1 compared to more current lines - Henkei/Classics/United have similarities and variances.
For g1, a figure could be really expensive due to the scarcity of the figure - e.g unbroken Bristleback, Japan only released G1s. Popularity of the brick - Bludgeon. It could also be because the Item is known to be sought after so the sellers sell it highly priced, or in piecemeal - e.g. Fort Max.
Similarly newer items are expensive due to scarcity/ limited release - Henkei/ United - for Japs only yet sought after by non Jap collectors so with only a few pieces these are priced higher. But, arguably the QC is better so they charge more as well.
I have seen prices go high before and sometimes be on hiatus and plateau but with the advent of re-releases - eg Encore et all. Some figures go down in price - not much but still knocks a few dollars enough to make the figures affordable. Of course there is still some persistence on high pricing for really proven vintage pieces. KOs on the other hand are a bane to pricing of TFs.
G1s get reissued now for less (Encore) but the newer figures, well... unless these were to become classic pieces themselves and if the molds survive would end up in the future as highly priced as G1 TFs and could remain that way. Unless reissues ( quality ones okay - Im looking at RtS Rodimus while shaking my head) flood the market again for sought after figures matching the demand with enough supply. but it all boils down to not enough supply = higher demands = higher prices for few supplies left.
Wanted AM partner Vanguard, Myclones Dirge, G1 Victory Leo, e-hobby Dark scream ( the black version), e-hobby Magnificus
Parts- AM partner Basher-side guns, G1 Actionmaster Elite Windmill's blades[I][B]