Back off topic...
I don't see anything comical or by implication self-contradictory about the statement that Stonehenge costs 120 hammers and grants free monuments.
Let's say there's a coffee shop that will sell me ten cups of coffee for $1.50, and I get my eleventh coffee "free." We could say my eleventh cup is not really free, because in order to get it 1) I've had to become a regular patron of this coffee shop; 2) perhaps, pursuing this meager incentive, I've bought more coffee over a few weeks than I would have otherwise; 3) I've had to keep track of the damn punch card; and 4) I've spent $15.00. We could also say that I've simply payed a slightly reduced cost for 11 cups of coffee, and that this even this slight discount is reduced in value because it's been deferred as opposed to being applied purchase by purchase.
But of course I can be aware of all this and still ask for my "free" cup of coffee; all I mean by the word is that, at this particular time, I don't have to lay down $1.50, as would be typical, for this particular cup of coffee. I can legitimately and without cognitive dissonance say "free" here even even though I'm perfectly aware that getting the "free" coffee has had a cost.
The word "free" is used all the time in this limited sense, and the reasonable statement that, in civ or otherwise, "ain't nothing free" (any supposedly free benefit comes with investment cost and/or opportunity cost, and maybe there are strings attached) doesn't really contradict that usage. The meaning of "free" is context-dependent, and varies in either case.
So while I agree completely with the overall point about the need to take into account opportunity costs in the game, to make this point by attempting to eradicate a well-established understanding of the word "free," one which is used sensibly and consistently all the time in natural language, is a Quixotic quest.