The resource modifier should be avaible for all players, exstra money for AI is not a valid AI addition but can help poor AI's.
And it can be fun to have exstra money overall or just to incourage different game mode maps.
I must strongly agree. It could create some very interesting game concepts where we have more resources available, be it single player or multiplayer. If everyone were to have the same resource multiplier as well, it would be balanced while creating interesting game "types". Furthermore, players who struggle a lot could find it more fun to simply increase their resource multiplier instead of hurting the AI. Besides, those who don't want their resources multiplied...simply...don't have...to use it.
On a side note, they should probably appear on the stats screen when the game ends, then, so if a player got a really high score because of that resource modifier, it's clear why.