Our rule of thumb when cruising is to factor in 50% of your actual cruise package for personal expenses onboard i.e. drinks, shore tours, drinks, gratuities, gambling, souvenier purchases, etc.....
Others may say more and some less... it really depends on your spending habits and budget. Drinks can range from $4.50 and up for beer....$6 - $7 and up for alcohol and or wine by the glass.....also remember that drink purchases (including sodas) will incur an automatic 15% gratuity added to the bar bill.