It's a question that's both externally determined, e.g. "What does the market bear?" and also based on someone's own perception of their worth. It's hard to disentangle the two but very important in order to maximize revenue. What's more the objective question of what the market will bear is often colored by my own perception, which is a recipe to leaving money on the table.
Weinberg's advice is to set your rate such that you'd feel emotionally neutral whether or not you get the gig. If you over price (by your own feeling, not by what the market says) and you get the gig, you'll be stressed to deliver at what you think that price level should be worth in labor. If you under price (by your own standard, perhaps because you think the market will not accept more) then you'll be resentful for working below what you're worth. Having been a consultant myself, I can only advise everybody to challenge the own perception of what the market will bear.