65mph in a 55pmh, seems a bit weak, considering the preceding limit was 65mph, which you were traveling at.
However, I am inclined to suggest that you avoid using the argument that "I didn't see the sign..." I suspect most judges (if not all) would conclude that it is your responsibility to know your surroundings (and the includes posted limits) regardless of the circumstances.
While not really practical (from a time and/or expense standpoint) perhaps you could argue the speed limit was set unusually low, provided that a speed survey bears that out: How Should Speed Limits Be Set?
Could one invest the time to conduct one to scientific standards to which the court would give consideration?
Some time ago, I received a citation for 28mph over the posted limit on I-95 south in the Ft. Lauderdale area. A good friend in the area suggested that I contact a traffic attorney (they're abundant and extremely competitive), which I did. For a nominal fee, I was able to have the citation amended to my favor without the requiring of my appearance in court (as an out of state driver, that was worth the price of admission alone).
For a painless solution, that would ultimately what I would recommend...I think you'll be extremely pleased with the outcome.
Just Google, one of the following:
I wish I could offer you a more inspired answer, but investing $65 in this case, is a very small price to pay, relative to the potential of the outcome. At least one of them offers a "guarantee."
It'll certainly be less expensive than $500 and may only take the time of a phone call.
Good luck and let me know how it turns out for you.