IMO,the fact that the speed limit was lower on one side of the road than the other doesn't necessarily mean it's a speed trap.It could be due to a curve in the road,a nearby hill,or off-ramp or on-ramp,etc,etc.
But as for the "fine" itself,it's clearly BS.
Any time you get a speeding ticket where you have reason to believe that the posted limit is below a safe speed you should ask the officer and the court if they can provide you with a copy or a reading of the last official traffic survey on that stretch of highway that supports the lower speed as necessary.