Short answer: it depends.
Long answer: you need to contact Splunk Sales, your account manager or a reseller in order to get a custom quote. The price is usually linked to your existing Splunk Enterprise license and it normally represents a percentage of this. For instance, if you paid $1,000 for a 1 GB Splunk Enterprise license and now you want Enterprise Security, it'll probably be something like $250., which is 25%. You have to add annual support on top of this of course.But this is just an example obviously. The bigger your Enterprise license is, the more "creative" Splunk can be with your quote, as happens with almost anything in life.
PS: if you are using Splunk Cloud, I don't have a clue how much would it be.
