Here's a proposition...

N = Normal cost to cause revolt
W = Most expensive wonder (to purchase with no production shields)
Y = Cost of other wonder (1)
Z = Cost of other wonder (2)
A = Cost of other wonder (3)
B = Cost of other wonder (4) etc...

Incite revolt (causes incident)
N + .25W + .1(Y+Z+A+B...) = total cost

Subvert city (no incident)
2N + .25W + .1(Y+Z+A+B...) = total cost

This way in your example (without any other wonders in the city) would cost 67 + 400 = 467.

Another example: You want to subvert London (a revolt cost of 2500). The Cure for Cancer wonder and Apollo Program are in London.

Total Cost = 2(2500) + 1200 + 480 = 6680

N = 2500
W = 4800 (Cure for Cancer)
Y = 4800 (Apollo...)
Z = 0
A = 0
B = 0

Constraint - wonder must be "in affect".
(Leonardo's Workshop will not be considered in formula after discovery of Automobile)

You may have also noticed that in the subvert formula I multiply before adding. I think that it makes more sense because a city with five "working" wonders would be ridiculously expensive if the adding took place first.

Please review this and comment...