I play Idle clans, the value of a forum gold is actually around 140 fg per 1$ based on steam gift cards $100 to FG = 14,000 - 13,500 and forum gold is a currency that depreciates in value as an infinite amount can be minted, this site's pricing on forum gold has never changed to purchase more, however what is important is the actual market not the set value in $ to purchase forum gold. items in games and outside of the site are worth more $:fg than purchasing it from the actual market shop. if you can't understand that, then go read a book on economics ;)
The first part is interesting with using steam gift cards. That's certainly not a route I've investigated and will see what info I can find on. The deprecation of FG doesn't seem as important. If we have to take that into consideration we also need to take in consideration that Idle Clans gold is being vastly printed each day by various methods especially with platebodies. How are you converting steam gift cards into forum gold? The other thing to note, as you pointed out I haven't been in here long. But if the price to purchase fg doesn't change (only place I see to purchase it is via forum gold help and click to purchase), then that makes the situation worse. If true that means buying fg with $ becomes less and less viable. The value of buying items in game with $ and selling on market changes with the market.