Involving money would make it too complicated I think. As a conditional border you also do not spend the money.
Not sure i agree about money making it too complicated.
An other idea is to use the beacons as a condition before an English ship can bombard. There are twice the number of beacons from the number of ships so if one of the two closest beacons to a ship is lit it can bombard.
I do like this idea...it ties in well with the beacons use....but then there has to be no disadvantage given to the Spanish because of it.