If I develop an app/add-on that allows my application to interact with Splunk, is there a cost for me to have it available on Splunkbase? Does Splunk charge users or developers for the use of apps/add-ons?
Yes, it's free. You can host your app on Splunkbase or externally. See Options for submitting content in the Working with Splunkbase manual for details as well as links to developer documentation and information about certification.
There is no cost for putting it in splunk base
Also, 99% of the apps are free and do not require cost to download.
You would need however an approval from Splunk app team to upload an app.
http://dev.splunk.com/view/app-cert/SP-CAAAE24
Also check the AppInspect tool that checks your app:
http://dev.splunk.com/view/appinspect/SP-CAAAFAM