Most coolers that stop cooling need a part, not a new unit: a pump, a valve, a belt or a set of pads. Those are repaired and the cooler is tested on the roof before we leave.
Replacement makes more sense when the cabinet or tank is cracked through, the motor has failed on an old unit, parts are no longer made, or repair quotes keep coming every summer. In that case the options are a like-for-like new evaporative cooler, or changing to refrigerated cooling that also heats; our evaporative to refrigerated cooling page explains that path. Our evaporative cooling installation cost guide explains what moves the price of a new system.