This is a Splunk Cloud DDAA question.
I don't know the cost required to use it.
Is there an option contract fee?
Are there any costs associated with saving logs?
Here is a page that explains the basic pricing plans.
There is also a Splunk Cloud pricing FAQ.
And here is the AWS Marketplace pricing page.
Searching for this gives me a couple of pages, but it feels like this blog says it all:
There's a chart in the middle that lays out what it costs (which is essentially "It costs whatever your Amazon S3 buckets cost you").
You could also talk to your sales rep - they'll be be able to confirm this, or give you different info if required!