should I replace one or both and how much should ti cost?
Honda CV shaft replacement on 1999 Honda Civic
Go to http://repairpal.com/honda-civic-1999 and put in your info and if only one CV Shaft is bad no use replacing both.
1 more answer
Since the drive axles parts are expensive it makes sense to only do what's needed. It might be a good time to check the CV boots to make sure that they are in good shape. If there are any cracks or leaks you should replace them.