An interesting question, indeed.
Assuming that the carriers come in at £4 Bn each (as is now looking likely), and assuming that we went ahead with the originally planned 150 aircraft JSF deal....
No-one's willing to say how much JSF will cost us, but looking at present US flyaway costs, you can safely assume that they'll be at least $100 m each, so that's $15 Bn (£9.2 Bn) for the jets. But that's only including a unit flyaway cost for each JSF, and so you must add a +Y
So the CVF and JSF come in at about £17 Bn +Y.
(If you can get two CVF and 150 JSF for £4 Bn, or £8 Bn then I'll take 'em, and I'll swallow every possible objection. That would be such good value for money that niche capability or not, I'd strongly support them. Alternatively, restore the RAF FJ fleet to a proper size (18-24 squadrons) and I'll support them even at £10-12 Bn).
Do you include R&D on Typhoon, or not? Do you take off the export earnings from the Saudi Typhoons?
Including R&D, and excluding any export earnings, the last official, NAO audited total cost of 232 Typhoons (more than we're getting) was £19.3 Bn.
Seven squadrons of Typhoon, or two carriers and four squadrons of JSF?
I know which seems to fill a core UK national defence requirement. Which is a 'must have'.