Travel, increased recruiting, facilities enhancements,
advertizing, salaries, more scholarships for other sports, other sports facilities, etc, etc, etc,
Clearly Sunday play is not an option for BYU and they would never accept unless that was a non-issue. There are, however, significant additional expence associated with being in a bigger conf.
BYU's athletic budget would be, very likely, the smallest in the Big 12. Utah's is the smallest in the PAC.