Looking at wikipedia, some of the early ones were held at the official (UK) residence, former official (Germany) residence, and a state guesthouse (Japan). They're often held in historically significant places, sometimes resorts, a couple times were museums and one was a library. Most have been held at some resort or other (likely because of the accommodation requirements), sometimes ones with historical significance, sometimes not. Once or twice it was held at a convention center.
I suspect that as long as Trump doesn't hold it at one of his resorts, nobody will bat an eye at it. The only real common threads here as far as 'normally hosted' are a major city (usually, but this is likely a consequence of logistics than any sort of precedence), a resort or other (most of the time), and historical and/or cultural significance of some sort (most of them). There's not a huge lot of precedence and logistics and security requirements are really the main factors driving it here.
I'm not discounting that the whole thing was a cash grab and a cash grab alone, and I'm not suggesting it might not have been that as well(especially given the poor business that golf course has had), but the whole thing just stinks of paranoid control freak more than greedy bastard. He wants it at a Trump property because he knows the Trump properties. He'd rather put it at some place he has some certainty of the quality than let it be handed off to someone else he doesn't trust. (And at this point he probably doesn't trust anyone, except maybe his kids.) His image is much more important than profit. That's pretty clear at this point and he just wants to be sure that a big event like the G7 doesn't lead to his image being tarnished any.
That's probably a good chunk of his thinking. He's also said that he still thinks of himself as being in the hospitality business (which is fine since that's the background he's drawing on) and wants to give the best, which is also totally fine, but given his background, you'd think he'd be able to suss out equally grand places to host it at besides his own resorts and what places he can trust the most other than his own thing.