355 F1 Valuations

Every month or so I check the valuations at the various sites for a number of Ferrari models. Today when I checked the NADA site and looked at valuations for 355 F1 cars, there was a marked upswing of the values, especially for the spyders. The increase was primarily in the "High Retail" category and represents a >7% change from the last time I checked, which was about 5 weeks ago. Any opinions on what might have changed the values so much? How are the NADA values calculated? From actual dealer sales?

 

I agree the spyders will always command a premium , but appples to apples the same 1998 F1 Spyder went up over $20k in NADA "High Retail" valuation in just 5 weeks. Of course, that is the model that went up the most, but here is an example of a few other F1 valuation increases in the last 5 weeks.

1999 GTB F1 - Up $3,600

1999 Spder F1 - Up $15,500

Does anyone know how NADA comes up with their valuations? It just seems to be a big jump in such a short period of time.