Hi @valdie,
It really depends on the features you'll be using, the volume and number of users...if you're still a beginner and this is for a non production environment start with a free version to get a hang of things.
If that's not the case have a look here for the pricing :
https://www.splunk.com/en_us/software/pricing.html
You could also opt for a cloud approach which would be much easier to maintain.
Cheers,
David
