Is it possible that those prices are for the monthly supply of pills in those dosages? ie. 1 month suppy of 10mg as opposed to just 10mg?
No. I'm sure that Sirtris/GSK would have filed extensively on this. There's no way Cayman could be selling pills. Even if it were a month's supply, it would still be insanely expensive! In the mouse experiments it was used at 100mg/kg, which does seem to call the "1000 times more potent" argument into question. These tiny quantities are purely for research or analytical purposes. The bulk price would undoubtedly be much lower, but you'd probably want to tell them it was for the little fat mousies.
Well, the suggestion that SRT1720 might be being sold only for research or analytical purposes might explain how Cayman could sell it without a license from Sirtris/GSK.
It's certainly not patent infringement simply to build an invention that is patented. It is only infringement when it is offered for sale. If other companies want to make SRT1720 entirely on their own, and do not intend to sell it, but only wish to analyze it, it's not patent infringement.
Now maybe an extension of this concept is that other companies can also make an invention, so long as it is, demonstrably, not being sold for the commercial purposes intended for the invention, but only for analysis and research. Certainly putting SRT1720 at this out-of-this world price would be the best possible evidence that it was not being sold for the intended commercial purposes described in the original patent, because it would be impossible to sell for those purposes at that price.