Convertable GP vs XP costs

[DeAnno]

Yea...

I'd GUESS that the reason gold increases exponentially, while equipment cost only increases quadratically, is because they WANT item power to increase faster...I don't know why though

Never assume that something happened because people understood math when it's easier to explain it with people not understanding math. It's really a shame WBL doesn't scale quadratically like almost all the items do (even wands and staves when you consider Caster Level*Spell Level is going to cause quadratic effects).

It doesn't actually even scale exponentially: it's doubling time at early levels is much quicker and then it slows down as you level more and more, so its rather difficult to model at all. 10-20 follows one speed, 21-30 another speed, and 1-10 seems to be an almost random assortment of guesses.