"Poorly paid," meaning simply that the pay is low, or "underpaid," meaning that the pay is lower than deserved. 

"Thousands of poorly paid Russian medics and opposition supporters marched in central Moscow..."

"Seniors were fed substandard meals and cared for by overworked and poorly trained and poorly paid staff..."

"Walmart Again Holds Food Drive for Own Underpaid Workers"

"Tony Blair says politicians are underpaid"

"ILO Says Women Still Underpaid Around Globe"

The most direct antonym would be "poorly paid".

But you could also use;

Minimum wage earner

slave labour

low paid

underpaid

   The dichotomy  might be   Remunerated   or Unremunerated  if the amount paid is what was owed.

Badly paid is also admitted.

Poorly paid