I assume that you meant to say "I am hoping that in Charleston, the Palmetto P25 is NOT encrypted."
Encryption is done on a talkgroup-by-talkgroup basis, and even radio-by-radio basis. Sometimes you may hear a conversation with one side encrypted, and the other in the clear.
At this time, a relatively few number of talkgroups in the Charleston area are encrypted. You'll find most agencies are in the clear. That decision is typically made at the agency level, not at the system level.
Any agency in Charleston Co could opt to encrypt at any time. The cost would depend on the options that were purchased for their subscriber radios at the time of purchase. If they purchased the encryption option originally, the cost to implement it might be pretty low. If not, you're correct that it would be relatively expensive and time consuming to implement.
I believe that, unless something notable changes, most agencies will not soon encrypt, since they didn't opt for encryption when they migrated to this system 4-5 years ago. That might be wishful thinking on my part.