This isn’t an item that is being marketed at a premium on the strength of Byrne’s signature alone. It is not a regular signed edition, and their target audience is not collectors of £20–£30 signed books. As has been pointed out, there is already a signed standard edition to cater to that demographic.
Instead, this is being marketed, via the inclusion of the limited edition signed print on high-quality paper, as an art piece, with a gallery-level price to match. Whether of not one values Byrne’s visual art that highly is subjective, but the art market operates on just such terms.
Personally, I don’t find the drawing sufficiently aesthetically pleasing to justify a £750 outlay, but there might be some well-heeled fans who do. £250–£300 would seem a more realistic price point, IMO. But my opinion, or yours, doesn’t matter much in the grand scheme of things. If 50 people buy it, then the edition will be considered a success. Whether it will later hold its value on the secondary market is another matter entirely.