i'm a OG foilfag and promofag, i don't know how much but i know i've ridiculously priced stuff like playset of azusa kamigawa foil, playset of chalice mirrodin foil, playset of gemstone cavern & mine foil, playset of vial darksteel foil.. As of promos i have like 10 aetherflux reservoir, paradox engine and inventors' fair promos.
I'm also a sucker for beta basics, fbb and old foils basics.
Btw i believe promos foil are pretty darn undervalued, usually they are worth slightly less or equal than their standard set foil counterpart, but have only half to a third of the quantities availible online... BUT all the promos are handed down the first few weeks, while foils are still kept in sealed products by big stores and hoarders for the years to come. Also promos tend to hit the secondary market way more often than foils, especially for low end stuff.
So i think there is at least 4x more foils than promos of any card printed. Which logically would make them twice to 4x more expensive. Idk if the market will correct itself over time, but from my experience it seems like very slowly but surely, the promos start to creep up in value at a faster rate than foils.
I'll tell you a secret btw : promos from kaladesh are rarer than the kaladesh inventions, yet they are worth way less because of all the hype and advertising around the masterpieces (and the inventions also looking damn nice i admit).