Start with the local homebuilders association. Most of the larger counties such as Horry, Greenville, Richland and Charleston have home builders associations with staff who can help you with estimates on what a project should cost. It won't be an exact price, but it will give you a baseline for work.
Ask some friends what they paid for a project. This will give you reasonable parameters for what your project will cost.
Get multiple bids. Don't go with the first contractor that walks in the door. Get at least three contractors and choose which one you feel does the best work at the best price.
Ask contractors about specific costs. Make sure to get an itemized list of every part of the project. South Carolina law requires a contractor to give that list if asked.
Check the contractor's insurance and bonding issues. If the contractor does not have anything and someone is injured on the job, you may end up being liable.