I’ve never used a GETMAIN in a CICS application program. I have only used them in batch jobs.
I cannot give you a diffinative answer as to the cost of a GETMAIN and its associated FREEMAIN.
But <i>anything</i> that is done in a CICS program 10 <i>thousand</i> times has to be expensive. Reducing the number of anything that is done 10,000 times would speed up the program.