Personally, I have a formula that I use to give me a rough, roundabout estimate of what the potential traffic might be worth, which I use as a starting point. Generally the higher the client's potential revenue, the tougher the competition, but this also means I can effectively charge more.
Anyway, my formula is:
[Est. local monthly searches] * [Percentage of expected traffic] * [CPC]
A concrete example:
The keyword "chiropractor miami fl" gets an estimated 480 US monthly searches on broad match and has a est. avg. CPC of $4.23 - let's assume we think they'll get 20% of the traffic from page 1, which yields
 * [20%] * [4.23] = $406
Keep in mind this is a rough estimate of what their market dictates is the value of traffic based on PPC spending. This at least gives me a starting place to think about what I should charge the client to where it is reasonable for both parties to render a mutually beneficial voluntary transaction.
Thanks for your input.