Michael, in the corporate world, my proposed solution would be called "collusion". In the micro world of course measurement, who would care that measurers in a particular region get together and verbally agree to certain minimum fees? I don't see the SEC taking notice of our 3 or 4-figure fees when Wall Street and Fortune 100 companies are making an industry in shady practices and tax evasion. In my neck of the woods, the major measurers got together a few years ago and compared notes and then agreed on nominal minimums. We all feel this protects us from the sort of thing you have encountered. No one is bound by these rates in any way. It is just an honor system. But it works.
Whenever I get an email from someone who is shopping for the cheapest rate for a certification, I always forward the message to my friends in the region so we can all be aware. It works. You may not always be the guy who gets the business in this scenario. But at least you know you are not being undercut by someone. Usually, you get the work if you were the first one contacted.
Another factor to consider is the quality of your product. I find that my clients to a person all prefer the 4-color electronically-drawn maps I provide them. I always send a couple of map samples with my proposals. I would say conservatively that my conversion rate from first inquirers to satisfied customers is north of 99% over the last 4 years.
Another technique that I employ is asking them to clarify details of their route for me on my first response. I take their MapMyRun or turn-by-turn directions or whatever they give me, and I go do a quick GMaps interpretation of their course and send it to them for "confirmation". I usually add a question about whether they need a contiguous Start-Finish, and a comment that I will probably need a little more real estate to get to the accurate distance. I attach my agreement and ask them to send me the 50% deposit I require.
Not to brag or complain - but so far, this works EVERY TIME.
Best wishes and good luck.