So I understand that you request a cost per transaction column and it makes sense, but that link sucks.
That assumption is incorrect. The open source databases are not included due to nobody having spent the time and money necessary to be included there. Sun did it for PostgreSQL many years ago and got decent results.
TPC is a non-profit that creates and publishes standard benchmarks that can be used to compare database systems. They had done so for over 20 years, and with widespread consensus among competing vendors (who outright hate each other) that the benchmarks are fair and representative of real work loads.
Mysql and postgresql have both been in the benchmarks published by TPC. They are rarely there because people simply don't spend the time and money to do the benchmarks. Back when Sun was providing commercial support for postgresql, they ran benchmarks for postgresql and submitted them. Anyone is welcome to submit benchmarks of any database, running on any OS and hardware combination.