Gold as an addition to the existing wood "currency" will only be interesting if you get it through other means than how you obtain wood (lumber mills, which already are bland enough). Otherwise it'll be just the same in a different color.
I have not figured out the whole map, yet, but maybe you could have TINY amounts of gold for kills and small side quests, where you have to go to far away places in the map with your mercenaries and do stuff. Things like "Oh no the power source has shut down we cannot run lumber mills anymore" and you have to turn some generator on again or repair it. Maybe have an engineer unit for that? Then you would get gold from the "Fortress Administrator" (or whatever) as a reward, which you can use for small non-essential but useful devices, like more Aura Tower Upgrades.
Problem with that is that monsters get really strong really fast, and mercenaries will just die on encounter, and don't really deal notable damage alone. Maybe have a tough pricey melee unit to act as a tank while the engineer is on his way? Would have to make sure they don't replace walls, though in the end you use units as sacrifices so your walls don't get raped too much anyway. (On Easy, that is. No idea how to beat any harder difficulty, yet. Every time we tried it we just called it ridiculous and play on easy.)
The map uses too huge values in general anyway. You could just divide all health, damage and other related values by 10, so it'll not look so exaggerated anymore.
Edited by -M-, 27 October 2009 - 08:25 AM.