Generally they're twice half. That's a joke.
Transient dockage can be over $5 per foot in places like New York city and as little as 50 cents a foot including electric and water. I'm going to the Black River harbor on Lake Superior. The US Forestry service owns the harbor and my boat works out to 50 cents a foot.
I usually anchor out, take the free bulkheads, or do moorings. But I do occasionally do dockage. $2 per foot is near my limit.
There is a growing trend to make marinas sound like resorts. If that's what you want, prepare to pay for it.
To answer your question, I think average transient dockage would be around $2 a foot.
Ted