Well, if you don't know how it should work, then you shouldn't expect him to know either. So don't be upset if he doesn't follow how you think he should act, and in the end it isn't worth losing a friend over a few bucks and a misunderstanding.
IMHO, proper etiquette depends on how the invitation was made. If there was no mention of money at the time you invited him, then my assumption is that you should be prepared as the "host" to pay for your "guest" (and he may or may not offer something). Remember he may not know what it costs, and you don't want to put him in an awkward situation where at the end you ask him to pay half and he doesn't have enough money to cover it. And you certainly don't want to be upset because he didn't know you expected him to pay.
If you have the expectation that he should pay something, then talk to him and let him know ahead of time how much it usually costs, and ask if he wants to split the cost or maybe tell him you can pick up the cost this time and he can get it next time. Allow him to make an attendance decision based on his cost.
Be clear up front and you'll avoid problems down the road.
Good Luck.
