Timestream multi-measure pricing

I have a timestream table with these columns, inserted with multi-measure type records:

system (varchar) measure_name (varchar) time (timestamp) MEM (double) CPU (double) DSK (double) SWP (double)

most of the records have MEM,DSK,SWP set to null (empty).

How the size of the record is calculated?

a record with all the data system=rpi, measure_name=v, time=2023-07-31 12:38:15.474000000, MEM=16.8, CPU=9.7, DSK=11.6, SWP=0.1

and a record with some NULLs system=rpi, measure_name=v, time=2023-07-31 12:38:15.474000000, MEM=-, CPU=9.7, DSK=-, SWP=-

have the same size/cost?