First things first. If you short out the CHT wires, you should get ambient temperature. To do this, take the D25 off which has all the CHT and EGT connectors, and make up a temporary connector that shorts out at least one of the CHT channels and one of the EGT channels.
Both the EGT and CHT should read the same. They read "ambient," but this is ambient inside the unit, so it could be 10-20 degrees higher than outside if the D180 is on for a long time.
You can't short the wires at the CHT connector and be 100% sure, since the wires leading up to this are thermocouple wires and you're still making a thermocouple if you short there.
Tell us what you find and then we can work from there.
I assume if we told you the repair would be $250, that this unit is more than 3 years old? If it was purchased used, are you sure the CHT's are set up for your engine and probe types, and aren't set to Rotax or something?