cave crocodile eggs, quarry bush leaves, and forgotten beast meat/fat are very valid. The value of prepared meals is in their stack size, not their individual worth.
I'd say it's the combination of the two.
As far as i can determine, prepared food value goes like this:
10*quality (entire meal) + ingredient value*quality _per ingredient_. The result is the value of each item in the stack.
For a simple example, let's take a roast made of nothing but lard (a.k.a. pig tallow), all work exceptional:
(10x5 for the whole meal) + (1x5, four times for the ingredients) = 70 per meal, stack size four, 280 for the full stack.
A roast made from nothing but duck eggs (or eggs of another "mundane" bird) would have the same per-item value, but the stack would be much larger, 50 would be quite normal, for a value of 3500 for the stack.
Counterexample for high-value ingredients with lowish amounts: roast from dwarven sugar and dwarven flour:
(10x5 for the meal) + (20x5, four times) = 450 per meal, let's say ten meals in the stack, so 4500 total value. That's already a bit more valuable than the egg roast.
And now to combine the two: a roast of ordinary eggs and _one_ bag of whip vine flour:
(10x5 for the whole meal) + (1x5 thrice for egg and 25x5 for whip vine flour) = 190 per meal, a realistic stack size of 40 results in a full stack value of 7600. More than half of the value is generated by the flour, which could be a single unit.
A quick look into my larders showed as possibly the best combined meal one of three parts dwarven sugar and one large stack of guineahen eggs, coming up at 780☼ per meal in a stack of 20 - the entire stack is valued 15 600, the value of 130 masterwork granite mugs.
I usually don't pay with food, though, it just feels too easy. I'm partial of clay and metal crafts. Those have nice value multipliers, crafts are the best use for soft metals like gold anyway, and they're a good reason to dig for magma.