Do I Need to Pay Separately for Bringing and Stitching All Data from a CJA Dataset into CJA Connections?

I don't know much about CJA or the billing... but I believe like Adobe, you pay based on server calls (or rather "rows" that are sent from the Experience Cloud to CJA), not the volume of data... (I assume that there is some cost associated to the calls going to AEP??? I am kind of the wrong person here, since I have never seen a CJA contract)

It depends on how many rows you bring into CJA. 

Usage limits are regularly monitored and enforced by Adobe. “Rows of data” means the daily average rows of data available for analysis within Customer Journey Analytics.

For example, let’s say your contract entitles you to one million rows of data. Suppose that on day 1 of using Customer Journey Analytics, you upload two million rows of data. On day 2, you delete 1 million rows and keep your usage at that committed maximum (that is, one million rows of data) for the remainder of your License Term. Depending on your contractual terms, you may still incur prorated over-usage fees for day 1, since you exceeded your “rows of data” license entitlement.

Please see - https://experienceleague.adobe.com/en/docs/analytics-platform/using/cja-overview/cja-faq#overage

For more details I think you can reach to your Account Team and check the contract details.