This all depends on the accuracy of what the mechanic is telling you.
If it indeed is the temperature sensor, then your worst consequence is like
ly to be poor fuel economy.
On the other hand, coolant temperature sensor codes can be set for some oth
er, more potentially damaging issues.
If your coolant is low, the sensor won't read the coolant temperature prope
rly. Your engine could overheat and you wouldn't know it. Being an aluminu
m engine, it's very sensitive to overheating.
A stuck shut thermostat can cause a temperature sensor code to set. Again,
this can cause overheating, potentially damaging the engine.
Of course the good news is that the thermostat or temperature sensor aren't
particularly expensive to replace. If your coolant is low, you'll need to
investigate why, and the cost of repair will depend on the cause of the lo
w coolant level.
The reason I'm questioning the mechanic's judgment is that you've elaborate
d what the mechanic said the issue was and then asked for a price. Mechanic
s get paid to work on cars. If he knows you need a coolant temperature sens
or, why didn't he give you a price for replacing it?