On the day we arrived, we went to a CVS on the strip and purchased snacks, wine and beer. We ate granola bars each morning to skip the big breakfast buffet lines, and we had our own (very affordable!) beverages on hand in our mini fridge.
One great thing about Vegas is the open container rule, so you can bring your own drinks from your hotel room and walk nearly anywhere with them! You can also get free drinks if you sit at the tables or slots in a casino. We did occasionally buy drinks, but overall stocking our own fridge saved us a ton of money, because you can expect to pay $10-20 on ONE drink in Vegas.