I'd guess a lot depends on specific books and or publishers.
For the heck of it I checked a book (a Fantasy novel) on both Amazon sites...
Gardens of the Moon by Steven Erickson
US paperback - $6.58
US eBook - $8.99
UK paperback - £6.29
UK eBook - £5.98
So in that case the UK eBook is cheaper than paper, but not by much.
Unless one is going by the "list price" of the print book, then the eBook in the UK is 33% cheaper.