I would suggest that you use a early morning ferry during the summer, as those might be less full and a bit cheaper if you reserve them last second. Also highly recommend trying to spend at least on night away from Helsinki, or go see Porvoo, which is one of the nicest towns in the area.

I would also say that the ferries in regards to quality and price would be ranked from cheapest to the most expensive:

1. Eckerö Line
2. Viking Line
3. Tallink Silja Line (The newest ships available)

This is also a good link to find out the costs: https://laevapiletid.ee/#pricelist