When we spent 4 months aboard over this last winter, we were at SH Charleston City marina, on their "winter special" contract. Sure enough, that qualified us for an annual membership, so we got freebie transient slips and fuel at their cost. In our case, not too bad a deal, since we were able to stay at two other SH marinas on the way down, ditto on the way back, and three up here after we returned home.
The rate for that "special" was $103.25/ft, plus power at $250/mo for 50A. So the total was up near approaching what we pay for a full year at our home marina.
But I'm under the impression you normally have to actually be berthed in an SH marina, and can't just "buy a membership" unless you come across one of these special kinda deals.
Then too, SH marinas don't grow on trees even though they seem to be everywhere. Up here, there are five within 40 NM, and we went to couple of those for a weekend each. But one is hard to get into, one isn't a location we care much about, and boat issues hosed up our plans to visit the third. Another is about 80 NM south, and we hung there for weather once on a longer trip Summer trip... but it's far enough away we wouldn't normally think of that as a short-term destination.
-Chris