The NPG paid £27,600 for the sketch which Hughes gave to his friend Roy Davids (as opposed to burning it, which is what he did with most of his other papers before he died).
The drawing was done by Sylvia Plath at Cambridge University in the early days of her marriage to Hughes, in 1957. As it measures 21 centimeters by 12.7 centimeters, that works out at around (if we've got this right) £103.50 per square centimetre (now email us and tell us how wrong we are).
In a statement issued by the auction house Bonhams, Paul Moorhouse, the NPG's curator for 20th century art is quoted as saying
The gallery lacked, and had been very keen to acquire, a really compelling likeness of Hughes made from life.