I was just on Carnival's website where you can order the drink coupons and they are now $19.50 for 4 and are not good for the premium drinks or the drink of the day with the glass you get to keep. Others have said that you save about $0.25/drink with the coupons so it depends on how much you are going to drink on the cruise. Last cruise we went on we only ordered drinks once on one night, we are not heavy drinkers and free alcohol was served at the captains dinner reception and the welcome back party so we were set.
If you want to check prices on these coupons yourself then go to carnival.com and register with mycarnival. It will let you get into the purchases for on board section then.