According to the UiPath documentation, the RegEx Extractor is a data extraction method that uses regular expressions to define and capture data from documents1. The RegEx Extractor does not consume any page units, which are the units of measurement for the consumption of Document Understanding services2.
Therefore, the page unit cost per extracted page for the RegEx Extractor is 0.
References:
1: RegEx Extractor 2: Document Understanding - Metering & Charging Logic