Splunk doesn’t reveal their pricing on their website. Splunk follows tier-based pricing model. Bigger the license cheaper it is.
It basically depends on the volume that you're indexing i.e. how much log data you are sending to Splunk to process/store. Splunk is free to use if your indexing volume is less than 500MB per day. The enterprise license starts at $6,000 for a 500MB/day perpetual license or $2,000/year for a term license.
Splunk's Enterprise plan has a number of great features to offer - no limit to the number of users, searches, alerts, correlations, reports, dashboards or automated remedial actions. It's all included—at no additional cost.