The price of staying in the old part of Charleston varies by the time of year; you can get good prices in the off-seasons., but during high seasons it is unbelievably expensive, with rooms starting at about $300/night and going WAY up from there. If you wish to be close to the historic area but pay less, the closest group of better-priced hotels are the ones near the SC Medical Center campus; they are about 12 blocks from the northern end of the historic area, with highway in-between; those are best if you want to save money but will not have a car.
If you do have a car, the next closest are the hotels in Mount Pleasant, which is usually a 10-minute drive over the Ravenel Bridge from downtown; I say usually, because if there is an accident on the bridge it can take forever. Mount Pleasant is also about 15 minutes from the beach at Sullivan's Island, if you wish to do both.
There are also close-in hotels over the river bridge in the Ashley-Phosphate area, but I personally don't like them. Traffic in that area always seems bad, and there are some parts that are a bit sketchy; if I'm settling for suburbia, Mount Pleasant is much nicer and actually a quicker drive under normal circumstances. Folly is nice, but farther afield; more built up than Sullivan's and Isle of Palms. You have to negotiate the Ashley-Phosphate traffic to go between Folly and Charleston proper, so I wouldn't choose it if my primary objective was spending time in the historic area.
