About 1e "dirty" (leafs and stuff) to 1,5e cleaned and its tax free, price varies due how much is available etc. so i cant predict what price will be. And its not in summer but more in autumn when those are ready to pick. In summer you can go pick strawberries in some berry farm in south. For most money you should pick http://en.wikipedia.org/wiki/Rubus_chamaemorus but thats hard work and hard to find.
Wild berry picking is free for all in finland as long you dont go too near to houses and you can sell those withouth any taxes. There is many buyers and you should look what price they offer.