I just went through the United Airlines 4th-Quarter and Full-Year 2017 Performance.

United Airlines Reports Fourth-Quarter and Full-Year 2017 Performance - Jan 23, 2018

The CASM was not listed for Regional operations, only Mainline and Consolidated. I calculated it from the data provided:

Mainline CASM 12.59
Regional CASM 16.93 (34% higher)

Most of that cost was due to fuel; the regional fleet burns 69% more fuel per available seat mile than mainline. However even excluding fuel, the regional fleet was still 26% more expensive to operate per available seat mile.