Access to Stadia will be free with Stadia Base. With that you can buy games and play them on Stadia without a subscription. There are no Free to Play to games on Stadia Base. Multiplayer is free on Stadia Base.
Stadia Pro costs $9.99 / £8.99 a month and is optional. You get at least 1 game month and increases the games graphics and sound. You also get discounts on some games.