Most of the ferry companies' income derives from tax-free shopping. A day trip to Stockholm will cost about 100-300 mk per person, for groups of two or more. Alcohol and tobacco are heavily taxed in Finland and both can be purchased cheaply from the ferries. Ferries generally leave Helsinki twice a day, around 9 am and 6 pm. Prices vary considerably according to the time of year.
Weekdays are cheaper than weekends and prices are lower in winter than in summer.
You should book your journey in advance. Estonia Travel between Finland and Estonia has increased enormously since Estonia became independent. It is a short journey, only 80 km, and many Finns now make shopping trips to cheap and pleasant Tallinn. The journey takes only 4.5 hours by ship or a couple of hours by fast hovercraft, so it is easy to take a day trip. It is also possible to take a 23-hour cruise to Tallinn.