Have you try to use get-Counter
.
Here is the line that gives my network card bandwith usage :
get-counter "\\jpbhpp2\interface réseau(intel[r] 82567lm gigabit network connection)\total des octets/s"
But be careful you CAN'T use it like that because counters names are specific to the language of your system.
So you have to adapt it to your computer try :
get-counter
then
get-counter Get-counter -ListSet *