Actually, I rather like the soul-harvesting idea. It certainly fits with the theme overall a lot more than wagers (none of which would actually flow to you on the battlefield, pointedly, since the bets would be between the Gods elsewhere watching). The harvest of a perfectly reasonable local resource doesn't actually fit with gold.
Pointedly, most of the maps appear to be occuring on constructed arenas, anyway. Why'd there be gold to mine in there? And who's working in them? And where is it being smelted and/or conveyed to exchange as a currency? The more you think of it as a physical resource, the more it actually falls apart, which is a fate an ethereal resource doesn't have.
Hmmm, "Spirit" might be a better term than "Souls," in fact. More powerful entities release more spirit when you vanquish them. The gold mines become Spirit Engines, which harvest the ambient spirit energy of the area and channel it to whoever has established control over them. Spirit energy can be converted to physical objects (and back!) at the various Spirit Forges/Shops. Since it's an immaterial resource by definition, there's no reason to ponder overmuch where you carry huge heaping sacks of gold you pick up off that fallen archer.
As an interesting aside, this means you could come up with a mechanic orthagonal to mana in which the more Spirit you're charged with, the more you glow / coruscate / visually effect, and certain Skills might make use of that persistent charge, as an aura effect or somesuch, but only so long as you did not reify your Spirit. So, you get to strategically decide whether or not to go for items / artefacts or for Spirit Skills, introducing another strategic and customizable decision fork.