Upvote:5
It depends on the exact plan you get, but in general the most you should pay is about 6 cents per MB for roaming when inside the EU. Some providers may not let you roam on a prepaid plan so it's best to ask when you buy the SIM card.
Some providers may provide free roaming within the EU or the Nordics/Baltics. For example Sonera have a data plan for €24.90 for a month which lets you use the data in the Nordics and Baltics. You might be able to find a similar option for prepaid plans.
https://www.sonera.fi/kauppa/liittymat/nettiliittymat/prepaid-netti https://www.sonera.fi/dam/jcr:15386e12-dad3-4397-836c-44c919d73860/Sonera%20Wireless%20Surf%20Service%20Description.pdf