How do pilots decide whether or not to deploy a ram air turbine?
The reason why the RAT deployed is of primary concern. The RAT should automatically deploy when (depending on the design of the aircraft) you lose all primary electrical and/or hydraulic power. In these cases, an immediate diversion is necessary because you may be one failure away from losing control of the airplane.

However there are uncommanded RAT deployments due to system malfunction, or mistake. For example, the pilot hit the RAT button by accident, or somebody accidentally switched off both generators instead of switching on both bleeds. In that case, the problem is easily rectified and then the pilot needs to decide whether to divert/return or continue the flight.

For Airbus, they state


  The uncommanded extension of the RAT is not considered to have any additional operational impact. The RAT is able to be extended in all flight envelope conditions. If there is an uncommanded RAT extension, then this is indicated to the flight crew, and hence the appropriate maintenance action, i.e. the resetting of the RAT, can be performed.


Fuel burn is minor, I think its +1% on the A320 family. The main concern operationally is that a RAT is loud. They sound like a prop plane or helicopter, so for passenger concerns, you might end up landing and having it restowed.