The cost of mixed drinks vary by cruiseline, but they are all in the same ballpark. For example, we had an afterdinner drink on RCCL in 2005, and it was $8 (plus gratuity).
A drink on CAL last fall was $5.50 (plus gratuity). That was, however, the drink of the day in a regular glass, and the RCCL was in a keepsake glass.
Each ship has drinks of the day, and some have drinks of the day and martinis of the day and such. Prices for those are usually a little less. And, like I said, we got the drink in just a regular glass the second time. If we had gotten the commemorative glass, it would have cost about $2 more.
Hope this helps!
Chris