I haven't tracked it carefully, but no, I don't think there's any law that says fees have to be the same on a given route. However I doubt they will fluctuate day by day. I rarely pay for MCE but two times I remember doing so, I paid $81 on LAS-PHL and $46 on a longer route, PHL-LAX. (I actually paid $23 but in the second case I was eligible for a 50% discount for gold status, which didn't apply to the first case.)