The international service plan from AT&T for the time the four of us of us were in Finland would have cost a total $600
A more frugal option is to buy SIM cards once you arrive in Finland. You can get these at a convenience store (7-11 type store) or from a cell company store. We were walking through a shopping mall and found a convenience store there that had several brands of SIM chips. I bought one from DNA. It worked well.
All four of us bought SIM chips in Finland that gave us unlimited text, talk, and data and also Finnish phone numbers. The total cost was less than $100. We were happy with that nice savings.
Cell coverage in Finland is outstanding. We never were without service. Jen and I were hiking in a remote part of Lapland and we still had great cell signals.