There are three major telecommunications companies in Finland, each with their own prepaid card that you can buy from kiosks/online or dedicated stores.
The price is more or less the same, although discounts vary. For example, unlimited 4G internet up to 100Mbit/s is 19,90€/month for all carriers.
I’ve included the current price of a starter package online, that includes the SIM card and a pre top-up value in euros, includes 24% VAT.
Price from kiosks/carrier should mostly be the same but can be more.
DNA Super Prepaid
Currently 4,90€ (Starter top-up 7€) (order/information here) site partly in English
PS. If you want to pay as little as possible, I found that Saunalahti offers unlimited 2G internet for 12.90€/month (speed capped at 1 Mbit/s).
After using the 2G I could upgrade to 4G the next month for the same price.
Hope that helps,