For a beer you will be looking at around £2 - £2.50 and for a cocktail you will be looking at around £4.50 although they do have cocktails of the day which are reduced.
We regularly cruise with P&O and did the short pre-maiden voyage Ventura cruise. We always find that the drinks are about the same price, if not a little sheaper, than a normal pub in the UK. They don't sting you on the prices as you might expect.
Stella draught lager is £2.60 per pint, Boddingtons bitter is £2.30.per pint Most bottles and cans are between £1.90 and £2.50 depending on size and product. Cocktails [alcoholic] vary between £3.40 and £4.40 per half pint glass.Non alcoholic coctails are between £1.85 and £2.20.Hope this helps. Would add that these prices include the gratuity, no more to pay.I think they are pretty accurate.