I was just looking at marinas in Charleston last weekend.
Most marinas charge about $12-13/foot and have a 25-foot minimum. If you have a sailboat, it rules out a few marinas since many of the bridges that span over the rivers here are not drawbridges. There simply isn't enough clearance. For example, Bristol Marina is north of the Route 17 bridge (not enough clearance to go under the bridge with most sailboats). However, City Marina is just on the other side of that bridge and could accomodate.
The cheapest marina we saw was in North Charleston. It was a municipal marina called Cooper River Marina (
www.ccprc.com). They offered rates as low as $10/foot if you are a Charleston county resident and about $12/foot for non-residents (Summerville is not in Charleston county). However, when I looked, they had a waiting list (only about 4 ahead of me) for a slip. Also, the marina itself is "off the beaten path".