Alright, what you can charge as fees has to be substantially less in value (note value not cost) to your client over what he will gain from your work. Simple enough rule but hard to apply due to the shifting perception of value by geography and economy.
So when evaluating an opportunity, it's our job to evaluate how much value we can generate for the client and accept or decline the engagement on the basis of that. Tough choice when you are cash starved and need a gig asap, but if we break this rule we will have unhappy customer and bad referral, not to mention lack of satisfaction for your output too.