As asn aside, this system made me wonder: would nobles and the nouveau riche me exceedingly high level?
See, xp for gold isn't about having wealth, or being given wealth, or really even spending wealth. XP for gold is about taking wealth. Zero Sum, baby, for one to prosper another must perish. if you are a brutal merchant that destroys the fortunes of others, yeah, you've picked up some of that sweet, sweet xp to go along with it.
Unfortunately, as an npc, the rates of exchange aren't quite the same as for a pc.