I don't like XP for murdering. Generates murder hobo campaigns full of boring, soulless combat. Also brings you Paladins punching Orc babies for 1 XP, and other fun awful practices which are dubious in nature at best.
Secondly I don't like XP for picking up gold. It invites too many questions about if I pick up gold and hand it to the thief, do we both get XP... yadda yadda etc.
Thirdly I detest XP for gold spent when it's on training or the like. It's like you're asking for whoever the trainer's house to be robbed. Why didn't the trainer do the adventuring? More stupid questions with "just shut up" as an answer.
Fourthly I didn't even want to mention the version of XP I hate more than any other - XP for roleplaying or quests or other silly metagamey bullshit. I might make a rant about this later.
So finally I come to the conclusion. You can spend gold for XP in one of three ways:
Blowing it: Gambling, drinking, revelry, basically anything where the characters get absolutely nothing out of it (including meeting any tithing restrictions, paladins) and have no way to track it down after the fact. The exchange rate for blowing gold is 1 GP = 1 XP.
Charity: Specifically giving it away to people to help their situation with no expectation of anything in return (meaning your tithe is out again paladins). This increases goodwill in the community you're charitable at for a rate of 1000 GP = +1 Reaction. Over time this reaction diminishes as people forget about charity, other than those who were specifically the target of the goodwill. The exchange rate for giving gold away is 2 GP = 1 XP.
Investment: Specifically expecting to have a stake or any sort of control in a given community, business, etc. Buying your way on a town council, paying off a judge, dropping a bag of money at a Lord's feet so they will hear you (and moreover start a relationship with you), etc. The rewards here are intangible, nebulous, and hard to define, but nearly permanent until circumstances change. The exchange rate for investing gold is 3 GP = 1 XP.
And no, tithing isn't an investment. Tithes are 100% wasting money. That's why it's a hindrance. Buying anything specific and tangible for a character (a castle, a magic sword, etc) is also worth 0 XP. You do not get XP for killing monsters. You do not get XP for picking up gold or magic items. You specifically aim to buy XP when you get XP.
