Splunk isn't really in the same category of software as cPanel. From what I recall cPanel is a web based system management package. Splunk is a data analysis and alerting tool.
That said, according to our website:
"Download Splunk for free. You'll get all of the Enterprise features of Splunk for 60 days and you can index up to 500 megabytes of data per day. After 60 days, or anytime before then, you can convert to a perpetual Free license or purchase an Enterprise license to continue using the expanded functionality designed for multi-user Enterprise deployments. "
Sufficed to say, you can download and use Splunk, for free, on most average sized system log data sets.
As for diagnosing when your server is under attack, that's where Splunk would meet your needs. Check out my blog post from today where I show you how to setup iPhone alerts when someone attempts to login to your server with invalid credentials.
Feel free to ping me back if you have any further questions or would like an extended demonstration license of our product.