In terms of cost Eagle cost £15 million and Ark Royal cost £21 million. AFAIK there wasn't much inflation between 1951 and 1955, but Ark Royal did have 2 BS Mk 4 steam catapults, an interim angled flight deck and a deck edge lift, which Eagle didn't. In spite of that a third increase in cost seems excessive. Albion, Bulwark and Centaur completed in 1953-54 cost about £10 million each, but Hermes completed in 1959 cost £37.5 million. She was completed with 2 BS Mk 2 steam catapults, 6 degree angled flight deck and a deck edge lift. Hermes also had much more advanced electronics including the Type 984 radar, the Comprehensive Display System and data links, but a 300% increase in cost still seems excessive. By contrast Victorious, which was rebuilt to the same standard as Hermes "only" cost £20 million.
Meanwhile Centaur was fitted with a pair of BS Mk 4 steam catapults (1956-58 IIRC, which is why she missed Suez and) which is why she was retained as a strike carrier and the newer Albion and Bulwark were converted to commando carriers. I thought Eagle was fitted with steam catapults in the 1950s too, but the last time I checked I could not find a reference.
Eagle was fitted with more powerful BS Mk 5 steam catapults, a fully angled flight deck, a Type 984 radar, Action Data Automation (in place of CDS) and data links in her 1959-64 refit. Hermes was refitted 1964-66 and then (IIRC) was followed by Victorious for her abortive refit...