Orbitz has one for 297 per person on Northwest/United combo...
with a stopover in DC from Detroit to Charleston.
http://www.orbitz.com/App/ViewFlight...ts?z=gyxl&r=19
Was the 320 for both of you?
Savannah and Charleston are both pretty small airports, so you mght end up limited to the regional jets - if you can driver further out, you can end up with a bigger airport that Southwest, ATA, Air Tran, and the other discount carriers will fly into.