There is a two tier tax system in Charleston County. If you are a resident living here full-time, your taxes will be about 1/3 of what an investor or out-of-stater pays. Don't let this deter you if you are not planning on being in Charleston full time, because I still think you will find the taxes less than most other states. You can estimate your taxes with this handy link.
For TIDES condos, your tax district is #21 City of Mt Pleasant. Your assessment ratio is either 4% for full-time residents or 6% for part-time. Your taxes are based on your sale price ( appraisal value) and not what the previous owner paid (with the exception of that first year if they are not caught up on updating the records by closing). Also, remember that if you are going to be here full-time or you are over 65, you need to apply for these discounts. You will not receive them automatically.