Payment method should always vary, some guilds might not have a whole bunch of money but like R.V said maybe its a craftsguild making armor and weapons. Make a fair exchange with what they have as the reward, that could go for cooking guilds, etc...We will need those things to and if we are primarily just doing Mercenary work will we have that time to go off into those things?
@lynx - although this guild is neutral and only concerns itself with customers i still hold the right to go off on my own and fight my own battles under my beliefs and if people choose to join me thats just FINE!