Solmukohta's ticket includes the gala dinner on Saturday night only. The gala is a full buffet dinner with plenty of options -- both allergy friendly and vegetarian/vegan. The buffet includes free beer and wine.
The lowest cost one can survive with is perhaps around 11 € per day + snacks from the tax free shop. The boat also has refrigerator units which work on a 2 € fee. Remember that alcohol is tax free on the boats.
Also remember that we visit the harbours daily.
The cruise ship has several other restaurants and cafés where you can get both cheaper and more expensive food. These have their separate menus.
You can find restaurant descriptions and menus here: http://www.tallinksilja.com/en/web/int/silja-symphony-siljaline-cruise-ship#tabs-content-2
For example, cheap options for food can be a sushi set for 10.50 €, salad for around 8.50 € from Sushi & co, a steaming hot bowl of Salmon soup for around 10.50 € or a (veggie) burger for around 14/16 € from Cafe Mundo. If you feel like Italian, restaurant Tavolata offers meals from around €19 and above (cheaper smaller plates, starters etc).
Solmukohta does not offer prebooking for these places, but for the restaurants you can prebook once on the ship. Some of the restaurants can become full at times. Preordering buffets guarantees a spot.
You of course have the option to visit nearest supermarket and restaurants when the cruise ship is in harbour during the day. The cruise ship has refrigerator units that work on a 2 € fee per visit, so you can bring in your own stuff to supplement / replace some of the meals. And the tax free shop sells snacks and tax free beverages free of tax.
You can purchase additional meal packages to the dinner and breakfast buffets. Both the breakfast and dinners include a wide variety of food -- also allergy friendly and vegetarian/vegan. The dinner buffet includes "all you can drink"-style lager beer and "wine".
Breakfast: you can preorder breakfast in the buffet for days of your choice for 11€/meal.
Dinner: you can preorder dinner in the buffet for days of your choice for 38€/meal.
PACKAGE DEAL: If you order at least 2 breakfasts and 2 dinners there is a package deal discount of 8 euros.
You can also purchase buffet meals at the ship, but at a higher cost.