Experience For Gold (And Vice Versa) Doesn't Make Sense

In 1st Edition Advanced Dungeons & Dragons any treasure removed from a dungeon and "turned into a transportable medium or stored in the player's stronghold" is counted towards experience points earned. Not necessarily at a one-for-one basis: if it wasn't particularly challenging, the Dungeon Master can use whatever exchange rate he feels is appropriate. Examples include 4 to 1, 3 to 2, and even 20 to 1.

You also receive experience points for killing monsters. I've never played 1st Edition, but I can recall people that have on multiple occasions state that the majority of experience points were acquired by looting treasure and that this was intentional to encourage a particular playstyle of avoiding combat whenever you could and focusing on the prize.

I primarily grew up playing 2nd Edition, where awarding experience points for gold acquired was an option, albeit one that I don't recall myself or any Dungeon Master ever utilizing (not that I can remember if any of us were even aware of its existence).

For Editions 3 and up it never even made it to optional status, and while I can't say exactly why I can say why I would have omitted it from a design perspective: earning experience points for finding treasure doesn't make any fucking sense, because there's no correlation between a character's skill with, say, a sword being improved merely due to him hauling a treasure chest between points A and B. 

While you could use more logical methods to encourage a particular playstyle of avoiding combat at all costs in search of loot, and I think low hit points, a very slow recovery rate, and limited healing options are all perfectly valid, the 1st Edition Dungeon Master's Guide attempts to justify it through a sort of false dilemma, implying that otherwise, characters might have to engage in some sort of training or character-specific downtime activities.

Examples include clerics studying holy writings and praying, fighters exercising, and wizards deciphering scrolls. All activities suited for learning your profession, and perhaps staying sharp during prolonged periods of inactivity (or wholly unrelated activity), but I don't see any value for a fighter to have to practice the basics when he is routinely engaging in actual combat.

This makes about as much sense as someone who codes webpages for a living to have to routinely brush up on HTML basics. Though, and this is something I think I've mentioned before, I could see characters gaining minor amounts of XP for training during downtime. The amount could be based on level, so higher-level characters gain it more slowly and be further modified if you pay a trainer whose level exceeds your own.

Treasure should be its own reward, with characters earning both it and experience points by overcoming various challenges and obstacles. If you're concerned that characters will advance too slowly, then the solution is to reduce the number of experience points required to level up, increase the experience points acquired by defeating monsters, or both.

If you want to award experience points for class-specific activities, you could do that as well. 2nd Edition featured those, and while I can't remember if they were optional or not I do remember other Dungeon Masters and myself using them. This would also help justify varying XP tables by class: classes that expect to more frequently gain class-specific awards could require more XP to level up.

For example, fighters are likely to gain a class-specific award for slaying monsters more frequently than a wizard does from casting spells, since spells are a finite resource (at least in earlier editions). So you either increase the amount of XP a fighter needs to level up, reduce the amount of bonus XP from killing monsters, and/or give the wizard a bunch of bonus XP for each spell cast.

You could also include rewards for using skills, and depending on how complex you want to get even adjust everything based on class so, for example, rogues and bards gain more XP from skill usage than everyone else.

If you don't want players to have to murder-hobo their way to high levels, you can also simply state that XP for monsters is gained by overcoming the monster in some fashion. This is what we do in Dungeons & Delvers, so characters can opt to kill an ogre, sneak past, bribe him, or even scare him off. Whatever works, so long as the GM rules that you overcame the challenge and that the threat was legitimate (you would not, for example, gain XP by seeing the ogre in the distance and just walking away or taking a long way around).

As silly as gaining XP for gold acquired is, I've seen people genuinely pitch something even more retarded: earning XP by spending gold, as if engaging in capitalism will somehow make your character better at casting spells or picking locks. It's a monumentally moronic idea on par with usage dice, declaring that your own shield magically breaks to negate an attack, and spending gold on whatever you want in order to accrue Fate Points.

To be more than fair it doesn't make complete sense for killing monsters to improve every aspect of a character. Or rather, aspects of your character unrelated to killing monsters. It also doesn't make complete sense for a wizard to become better at casting spells by crawling around in a dungeon, especially if he casts no spells at all. I get that, but it doesn't mean you should double down on the absurdity.

Instead, you should rework the system so that it makes sense, or at least more sense, which shouldn't be terribly difficult given the nonexistent hurdle that is gold for XP (and vice versa).