There was another post in the thread regarding highest possible legit:
Highest per-ration value is 4,920☼ for an all-dwarven-cheese or all-dragon-meat roast. Cutting either roast with dragon tallow reduces the per-ration value to 4,320☼ and it's all downhill from there. As always, the maximum theoretical value is unlikely to appear in the wild without a user making a concerted effort (But this is DF - so we'll see a forum post about it next Wednesday). The odds of a Legendary+5 cook successfully getting a masterpiece quality modifier on all four ingredients and his preparation roll is about 0.5%.
Cutting any roast with a high stack-size food is a great way to boost the total stack value, though. A maxed-out stack of cheese roast (above) is worth 172,200☼. If you cut it with a stack of 25 dwarven syrup the per-ration value drops to 3,960☼ but the total stack value jumps to 336,600☼. Just goes to show that stack size matters.
That is from 40d though, so the values don't apply in the present version. Dragon/hydra meat was worth 100☼ per unit, as was dwarven cheese. Now that's down to 30☼ per unit of dragon (or roc) meat and 20☼ per unit of hydra meat or dwarven cheese. So the per-ration value for all of these roasts has plunged.
However butchery product stack sizes have gotten a lot larger. I'd imagine an all dragon meat roast would have a far higher max value than in 40d, if your world has dragons of max or close to max size. Wiki says that rocs can give up to 949 meat. Considering that max size dragons are 25% larger than max size rocs, I'll assume 1200 meat from dragons. That would mean a perfect dragon meat roast stack would have a value of (12*10 + 12*30 + 12*30 + 12*30 + 12*30)(1200*4) = 7,488,000☼
If someone wants to try to reach this in their own forts, I'd imagine a roc hatchery is the way to go. They're a bit smaller than dragons, but only take 20 years to reach max size rather than a thousand.
Edit: Just remembered giant sperm whales. Wiki says those can give up to 7048 meat. Approximating that to 7000, a roast with 2 stacks of dragon meat and 2 stacks of giant sperm whale meat would be worth (12*10 + 12*30 + 12*30 + 12*2 + 12*2)(1200*2 + 7000*2) = 14,563,200☼