It depends how long you're staying and how much you drink.
One night/two days will cost you $6 a day, or about three drinks (most amounts rounded to nearest dollar)
Two nights/three days = $4 a day, or two drinks
Three nights/four days = $3 a day, or 1.5 drinks
Four nights/five days = $2.40 a day, or 1.20 drinks
Five nights/six days = $2 a day, or one drink
At seven days and above, the cost per day is less than the cost of a single drink.
So, if you're going to drink more each day than the above 'chart', yes, the refillable mugs are worth 'it'. AND you have the mug as a souvenir to use every day when you get home!