I don't know if you're on your ship already, but here's my $.02. I caught a nasty cold on the plane down to our cruise trip - we had 9 days at WDW before the cruise, so by the time we reached embarkation day the cold was mostly gone, save for a nasty-sounding cough (my colds tend to linger in the chest before disappearing). If you think you're contagious, maybe get some Purell or the like for when you're walking through common areas of the ship (remember, door handles are the most germ-filled surfaces of bathrooms!) and be wary of any scary sounding coughs or sneezes in common areas - my DH swears he saw some CMs giving us the suspicious eye during the lunch buffet on the first day after I coughed in Cabanas (although our cruise was during the Ebola-in-the-USA panic, so that might have been a special case).
I know they had some medications in the gift shops, but we stopped at a CVS before hitting port so I couldn't tell you about the costs. I personally recommend Robitussin DM if you're a chest cold-er like me, and Advil Cold & Sinus otherwise. Have fun!