"ultimate difference" is correct and usable in written English.
You can use it when you want to emphasize a significant difference between two things. For example: "The ultimate difference between the two candidates is their policy on healthcare."
The Cleveland Foundation agreed to cover the ultimate difference in cost.
What is the ultimate difference between a man and a woman?
The ultimate difference, perhaps, is this: people with Asperger's syndrome can tell us of their experiences, their inner feelings and states, whereas those with classical autism cannot.
Unfortunately for I.A.A.F. officials, they are faced with a question that no one has ever been able to answer: what is the ultimate difference between a man and a woman?
Two misplaced passes in moves where the prospect of reward did not outweigh the element of risk led to 12 points for their opponents, the ultimate difference between the sides.
Dependent on an oxygen tank because of lung disease, John wheezes out abuse while his son unleashes "venom," the two of them rolling across the country in search of "reconciliation," the son seeking elusive proof of their ultimate "difference".
There are ultimate differences as well as a hierarchy of universals, the highest universal being existence.
Economists have found empirical evidence that covered interest rate parity generally holds, though not with precision due to the effects of various risks, costs, taxation, and ultimate differences in liquidity.
Equally, an ultimate specific difference is "primarily diverse" from any other, in the sense that such a specific difference "has a concept that is absolutely simple" (and thus cannot even overlap with the concept of any other such difference) (Scotus, Ordinatio II, d. 3, p. 1, qq. 5 6, n. 183 (Scotus [OO], 7 481; Spade (1994), 105)).
The White House said, "Our analysis tends to understate the ultimate price differences for insured and uninsured customers," because the government could not get data on rebates.
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