I've got more charts for you! This one is an historical comparison of Cab Franc to Cab Sauv prices. Cab Franc is in the foreground in black, Cab Sauv in the background in grey-green or whatever that color is. Cab Sauv led for a while, until 2007, when they seem to be trading off. The importan thing to notice here, in my mind, is that Cab Franc offers steady, similar financial benefits to Cab Sauv, making it an easy way to diversify.
Unfortunately, it is also a relatively weak way to diversify, since pricing seems to be correlated and it doesn't offer the same operational or biological advantages of diversification as, say, Sauvignon Blanc. Still, looking at these numbers, it makes me think that if you are going to plant a large block of Cabernet Sauvignon, you should diversify with about 5% Cab Franc. In Napa, Cab Franc has slightly more than 5% "Cabernet acreage-share". Statewide it's slightly less. Feel free to use this chart, however you'd like, but please leave in the attribution.