[Dixie]

First, are you sure they're carpenter ants? If you’re in doubt, take some to your local county agricultural service. They'll tell you what type of ants they are.

Be wary of the sales tactics. National companies, like Terminex, will prey on your lack of knowledge and fear. Did you buy the house new? If so, you can extend the bond the builder has. If you bought it from an individual, you can extend the bond the seller had. (I'm betting that NC law matches SC law, and your house was required to have a clear termite inspection/bond before it could be sold.)

But on to the root of your query. I have a reputable, local, company do an annual termite treatment/inspection/bond. Mostly so I can show the house has always had a termite bond. A nice selling point if and when I ever sell. As for price, as I recall, the cost is about $200/yr.

Personally I go one step further. I do my own semi-annual termite and pest inspection. For termites, look for dirt tubes extending up any pilings and skirting. The termites you'll encounter live in the ground. They travel through the tubes each day to reach their food source.
Search both in the crawl space, and outside the home.

And never transfer dirt from your yard to around your house. When the house was built they treated the build site with chemicals. The dirt in your yard wasn't treated.