I can't find anything in the help files. I have over 2000 credits in my account but get a message saying I don't have enough credits for a really straightforward join of a CSV file to around 900 polygons. Thanks.
join tool arcgisonline credit usage
Check out step 4 in this blog: Table to App – Part 1: Create Data | ArcGIS Blog
It goes into detail on how credits are charged for this process. Filtering unneeded fields can reduce the credit costs depending on your data.
From the documentation here
it may not be the join that is the issue but what you are using in the join
Retrieving data ...