(speaking as someone who used to be in the second hand book trade) I shouldn't worry - that's not a 'real' price anyway, Amazon is littered with daft numbers like that that people stick on at first when they're the only one offering the title, as if they could get away with it. It's only a kid's paperback isn't it and it's out of print so there aren't any new ones (unless, conceivably, they could get Ms Sutcliff / her publishers to agree to them printing a one-off, I dunno) but there are hundreds, thousands of them in existence. If you pick one that's 'very good' condition and it'll pretty much look like new (considering its age) anyway. (books are described as 'reading copies', 'fair', 'good', 'very good', 'fine' and 'mint' - and don't ever believe mint unless it's still in its shrink wrap)
public information broadcast over. god I'm boring myself now.