Darvond: Inferred value: The 30 gold isn't literal, but rather the exchange value of what parts of the spider could be sold in exchange.
Breja: That rarely works, because usually the spider will drop both gold and parts you can harvest and sell.
Honestly the spider dropping literal gold doesn't bother me much, though that's mostly because I'm just used to the cliche, and the fact.. well, those are game, a lot has to be "gamified" to be fun. A lot depends also on the particular game, the overall mechanics and tone it's going for. For example I actually like the way Bard's Tale (the 2004 one) did it, where items inferior to the ones you're already using were immediately exchanged for gold upon picking them up. But it's definitely not a system that would work in every game.
What does bother me, and that's just me being silly, because compared to the spider carrying around a wallet it's small potatoes, is the fact most RPGs default to using gold coins as basic currency. Like it's normal for every Tom, Dick and Harry in a medieval world to run around with gold in their puch, while actually most lousy bastards would be lucky to see a silver penny. Inflation in those magical kingdoms must be a bitch if a portion of measly stew will cost me a couple gold coins.
In tabletop D&D it's not solely gold pieces, that a Player Character would be carrying, and looking for as a loot drop.
There's Platinum, Electrum, Gold, Silver, and Copper pieces (pp, gp, ep, sp, cp respectively), as well as valuable gems, that a PC would have in their money pouch.
This is the currency coin table for
Forgotten Realms D&D 1 platinum piece
= 1 platinum piece
= 10 gold pieces
= 20 electrum pieces
= 100 silver pieces
= 1000 copper pieces
Those values are the same for
D&D 5e Most D&D computer games though, simplify things so only gp is carried and used as money.