$30 is very reasonable when it comes to boat parking. Nantucket is $75 a night if you can get at reservation. Catalina in $35 and up depending on boat size a site. Whatever the cost is I would include the charges and fees in the cruising permits. In the place of the nightly parade of toll collections. I would use on the water jobs and resources to inspect and repair the mooring and checks for cruising permits that could be as simple as a sticker on the hull.