Which particular MMP steam locomotive did you have in mind?
Let's have a look at some other 2-8-0s -
MOK Stanier 2-8-0 for £485,
Slaters GWR/BR 28xx 2-8-0 for £473
David Andrews GWR/BR 47xx 2-8-0 for £359
Martin Finney GWR/BR 47xx 2-8-0 for £422
DJH LMS/BR Jubilee 4-6-0 (no 2-8-0s listed!) for £540
£450 seems to be in the ball park.
BTW, the link above for the ex-Snowhill, now Roxey, Austerity 2-8-0 should be http://www.roxeymouldings.co.uk/product/686/7l18-wd-austerity-2-8-0/
The link in post #395 is for the 2-10-0.
Dave