I've read through the various responses and the only thing for certain is that each situation is unique.
For myself I have a plan for when I get my results (being optimistic that I will pass...
). Sorry if I am being redundant.
First off, research, there are hundreds of websites out there now Salary.com etc. as well as any number of professional organizations that anually poll thier members' salaries to get an idea of the market conditions.
Secondly demand, you don't have to go directly looking for other jobs to know whether or not your field is in demand. I know that my profession (Fire Protection Engineering) continues to have a high demand despite the rough economy right now because there are so few of us. I'm even considering printing out the list of help want ad from the UMD career center to show them what I am looking at. If you are a civil however things might be a little lighter in terms of demand due to the recession. Your ability to relocate will help in this situation because it removes the employer argument, "yes there is demand but if you stay with us you don't have to move."
The third thing is value, yes you may be doing the same job you were doing before you got your PE, but can the company charge a client more money for your services now because of your certification? (I'm betting most of them can.)
Commonality, (kinda parallels demand) are there other people in the company that can do your job or are you the only one? Even if your a civil, if your the only civil in the company they might be more inclined to pay keep you than to go outside and go after an unknown quantity just to save a few bucks.
Finally there are of course the intangibles, do you have other skills so they can plug you into something else if your field falls slack? What is your relationship with the company? What have your past work habits been like? What is your potential?
Hope this helps...