What a conditional DynamoDB transaction actually costs

Leonie opened an issue on the suite the other week with a question I couldn't answer off the top of my head: when you run a conditional write inside a DynamoDB transaction, does the condition cost you read capacity on top of the write? Or is it write capacity all the way down? So I went to the docs. They were... not clear. There's a line on the transaction API page about idempotent retries…