Get Memory Utilization Percentage 
Get-WmiObject Win32_OperatingSystem -ComputerName $computer | Select @{Label = "Computer"; Expression = {$computer}},
    @{Label = "MemoryUsage"; Expression = {100 - [math]::Round(($_.FreePhysicalMemory/$_.TotalVisibleMemorySize)*100,0)}}
Details
Returns the total percentage of memory ultization on your local or remote system.

Example
PS C:\> $computer = 'localhost'
>> Get-WmiObject Win32_OperatingSystem -ComputerName $computer | Select @{Label = "Computer"; Expression = {$computer}},
>>     @{Label = "MemoryUsage"; Expression = {100 - [math]::Round(($_.FreePhysicalMemory/$_.TotalVisibleMemorySize)*100,0)}}


Computer  MemoryUsage
--------  -----------
localhost          69
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