Your question is unanswerable unless you specify the cocktail (or its ingredients) and the nature of the spirits (well swill or premium).
From a simple G&T (e.g., Tanqueray vs Gunpowder) to a more complex Side Car (“no name” brandy plus triple sec and citrus mixer vs Hennessy VSOP Cognac, Cointreau and fresh squeezed lemon juice), the retail prices of a “single” will vary and a “double” (highly recommended since O tends to use a 1 oz. pour of whatever is the predominant spirit in your drink) will, at least, be double the cost of the primary spirit.
FWIW: When we first started with O years ago, we did shorter cruises (under 3 weeks) and would do the Prestige Package of “unlimited” (except for the topmost shelf of Single Malts and Cognacs) spirits and wine-by-the-glass (of mostly mediocre offerings). We now do mostly long O cruises of 1-2 months and do not buy a booze package. At the least, I mention this because keeping up the “pace” of daily drinking wanes with time (especially when you’re a geezer like me).
As regards booze package choices:
In addition to itinerary (number of segments and ports vs sea days), consideration is needed for your O Club status which, in addition to Captain’s Welcome events (one per segment), means one or more O Club parties and invited “officer dinners” all with complimentary spirits and/or wines.
You can upgrade to Prestige (unlimited) from the basic “dining wine and beer” package available as one of the O Life options for $20/person/day.
A couple of the above mentioned Side Cars each day would make Prestige (which also includes the gratuity) a bargain. But, you’d still have the mediocre wine choices.
As do many other O regulars, we bring our own wine - often a case to start (replenished along the way as possible). Note that the “six bottle limit” cited on the O website is a mere CYA statement and O has never limited how much personal booze one brings aboard (see pic) - as long as the rules are followed: spirits remain in the cabin as does wine unless an unopened bottle is taken to a dining venue or other public space and a $25 corkage fee, which includes tip, is paid. Needless to say, the “bargain” is clear when you compare the price of your own $60 Pinot Noir (plus $25 corkage including tip) to the same bottle purchased onboard at $175 (or more) + tip. Moreover, O will store/retrieve that opened bottle for you. Note that there’s also a “seven bottle” purchase program (from a list that is “discounted” but still not a major bargain) and the more promising “bin end” list with 25%+\- savings.
For onboard spirits, we take advantage of the above mentioned invited events, Happy Hour twofers, select drink specials at other events. And, on long cruises, the daily consumption definitely does vary.
Cheers!