Your suspicions are right, there is a rule about that. “Keep all discussions on pricing to general terms. Do not discuss specific prices for services.” I’ve replaced your numerals with X’s, which ought to be general enough.
As to your questions, not everyone does this, but I typically list a dollars per hour charge for anything that goes above and beyond the specifics mentioned in the contract. This helps alert the client to the fact that there will be an extra charge for additional work and also lets them know what that charge will be.
Are you selling the copyrights to the illustrations or just usage rights of some kind? It’s pretty common, as you probably know, with illustrations and photos to charge more for exclusive, long-term usage or ownership since it eliminates the possibility of selling additional usage rights down the road to others. Personally, I don’t do that, but then I don’t do many illustrations.