other possibilities are:
VikingLine: you get a 50% discount on the normal price, so it is for you between 5 and 20 EUR one-way
VikingLine: you get a 50% discount on the normal price, so it is for you between 17 and 25 EUR one-way
or if you do not want to travel via Helsinki, go down to Trelleborg (Sweden) and take a ferry of the company Scanlines. it is about 21 EUR one-way to Rostock, but maybe you even get a discount. ask for it !!
if you want to go by bus from Sweden to Finland you need to travel by train from Stockholm to Boden. There you change to the bus (a bus for this connection is always in front of the station. but hurry.)
Länstrafiken (Norrbotten): Boden - Lulea - Haparanda
Veljekset Salmela: Haparanda - Tornia/Kemi
You have to change the bus at the border, but the bus drivers are very nice and helpful and the passengers as well. You often meet other railers on this connection.
then take the train down from Kemi to Helsinki...
you need for this tour about 2 days !!!
but it is a nice tour !! you see a lot of nice nature !!
for train connections use: rail.cc/en/search-interrail-route
I hope I could help you, all the best,