So I did some poking around, and while the numbers in @Pyritie seem reasonable, I feel like there's got to be a way to game the system, at least given what we know about "rounding". So I did some playing around with some single items, and found that a single apple (3 rupee sell price) sells for 10 rupees when cooked, for a grand total "profit" value of 3.33x. Interestingly, if you cook 5 of them, you get an item that sells for 50 rupees, for a true multiplier of also 3.33x. However, you can do better :)
A single acorn can sell for 2 rupees, and due to some vagaries at the low end, still sells for 8 rupees when cooked, for a multiplier of 4x, which is the highest that I've seen so far. Interestingly, 5 of them sell for only 30 rupees, which certainly follows the right pattern (namely 5*2=10, and sells for 28 rounded to 30 rupees), for a profit multiplier of 3x, which is less than the 4x you get by cooking just 1.
The system favors cheaper items than more expensive items due to weirdness of rounding at low numbers. While this gives you the maximum margin of difference between the ingredient sell price and the cooked value that I've found so far, it's not fast to do, however. But you do make more total difference between sale price of ingredients vs. selling price of cooked food, that way than other things that I've come across. Perhaps a more in-depth way to figure it out would be good. If you can find 1000 rupees worth of acorns (500 of them), they'll sell for 4000 rupees, which is more than any other set of combination of items with an ingredient sale price of 1000 will sell for. Apples (and other 3 rupee base price items) will sell for up to 3333 rupees. 250 of 4 rupee base items will sell for about 3000 rupees cooked (when combined into 5 items, only 2500 otherwise), and everything else is between 2800 and 3000. Very time consuming (how the heck are you going to find 500 acorns, plus then cook one at a time, then vendor 500 items), but maximizes profits at the expense of time.
Generally, the post that @Pyritie made is correct, though.