I didn't itemize camp-by-camp but some estimations based on the Wiki base value and my records of what I normally earn:
As expected (compared to what I hoped) the bonus appears to be only on the base. So assuming a camp worth 1000 exp and a general with 3 skill books in fast learner you would get:
1000 + (1000 * .30) + (1000 *20) or 1000+300+200 for a total of 1500 for that camp. I was hoping that the 20% would be on top of the general skills but it's not (which is normally how other things like buffs are done anyway so fair enough).
I didn't try to drill in the details of Mary but I expect that the +20% is only on the base value of the camp.
As to whether or not it's worth it that's purely a subjective thing and how much you value the materials spent to activate the scouting post. I tend to be ok on book materials so for me more exp is good. With premium and 2 woodcutters I ended up approx. +44k which makes a few hinges and adv paper well worth it.