The 7 gram/day maximum dose I've mentioned allows a margin of error. It is based on the toxicology studies where 1000 mg/kg/day in rats caused kidney lesions, dehydration. At a five-to-one scaling factor (which is based on mass to surface ratios, not mass alone) that would be 200 mg/kg in a human. Cut that to 100 mg/kg for a safety factor. A 75 kg man could reasonably assume 7.5 grams a day is an upper limit to safe dosing.
Caveat: the study (referenced in the infinite resveratrol thread of 500 mg a day) studied rats at 1000 and 3000 mg/kg daily, and I think at 300 mg (not sure). No problems noted at lowest dose, 1000 mg showed mild kidney problems, 3000mg/kg daily more severe lesions and symptoms. All symptoms cleared up on cessation of dosing, except in the rodents that died (from other causes? Administration was by gavage.)
We know from the Auwerx's study that 400 mg/kg a day was safe for rats.
I believe Sinclair's $6800/kg cost refers to Sigma/Aldrich's catalog price for synthetic. An Indian manufacturer sells it for less than a third of that price. Plant extracts are cheaper still.
As far as I'm aware, the beneficial effects of resveratrol show a linear dose/response relationship up to the levels of toxicity. (I've never taken more than 2 grams a day personally.) A synthetic analog that was more effective at lower doses, with reduced toxicity, should show even more marked benefits than resveratrol
Lucid: (even though data I have seen shows true CR to slightly better Resveratrol).
Metformin shows better than resveratrol too. Combining metformin and resveratrol might switch on the CR genes that resveratrol misses. I think the reason more people are not using metformin is that is is a prescription drug.