I think jewellery which sold for possibly £200 (to afford two horses, the less valuable of which cost roughly £90) in the 1950s wasn't exactly limited in value. If you consider what the average weekly wage was at that time (between £5 and £10, depending on whether we're talking late forties - End of Term - or late fifties - Peter's Room), the Last Ditch was worth at least 20 times that. Hardly peanuts. Even if the younger children had exaggerrated its potential worth or usefulness in a crisis.