$letter='c:'
$valueToReturn="Used"

switch ($valueToReturn) 
{
"FreeSpace" 
{
Get-WmiObject Win32_logicaldisk -Filter "DeviceId='$letter'" | Select FreeSpace
}
"TotalSpace" 
{
Get-WmiObject Win32_logicaldisk -Filter "DeviceId='$letter'" | Select Size
}
 Default 
 {
 Get-WmiObject Win32_logicaldisk -Filter "DeviceId='$letter'" | Select Size,FreeSpace,@{n="Used";e={$_.Size - $_.FreeSpace}}
 }
 }