Light freeze damage (a pressure gauge spun out, a cracked fitting, etc) is one thing. A "hard freeze" is quite another.
If a machine loses a heat exchanger, it has usually had a pretty severe freeze. This can be caused by expansion of water within the system, or if "slush" is present, then the unit is started, and the "slush" travels to a choke point (like a check valve), and creates excessive hydraulic pressure that burst everything before the check valve.
Once a unit has had a "hard freeze" or a "slush hydraulic" condition, there can be failures anywhere that delivers water for months, sometimes years to come. We've had units come in with leaks in brass fittings, hoses, and sensors that work within the water delivery system much later than the original freeze damage.
I wouldn't buy a freeze damaged unit due to the unpredictability of future costs.
A number?
The depreciated value of the machine, less the cost of heat exchanger(s), water pump, pressure regulator, sensors, pressure gauge, and hoses.
I know that sounds awful, but 50 years of owning/selling truck mounts has made me VERY gun shy of frozen machines.