There is a guy on Ebay who had a red 912 with a 911 motor. It just expired, but I don't know what the bidding ended at. He didn't say in the ad what 911 motor was in it but it did have Carb's not CIS. Also, he mentioned in the desription that it was a great chance to buy an original early 911. Uhmmm
Anyway, my belief is that the more valuable the running gear is on the car, the more valuable it is overall as the car can always be parted for good money. For instance, a 912 with a nice clean 2.4S motor is going to be worth at least $6-8k just for the motor. A clean 912 body is probably worth at least $4-5k. Those two things alone would make such a 912 conversion worth at least $12k. A good way then to value a 912 conversion is to value the parts separately, and then add a few thousand for the fact that the car is together and running.