Akismet API Key for Multisite

Hi!

My question may be a bit redundant to some but I figure this is the best place to ask. While setting up my multisite, is it necessary for me to use the commercial Akismet API key or is there any way to avoid the monthly payment of $50? I set up a personal key and began using it per site I set up. (frustrating) Before I get too far with that, can someone help me understand how this process should go. Is there a way to set this up for the entire network so I do not have to do this for every subdomain? I am no tech expert. Just learning along the way...