So, if a defector would be worth more than £900,000, how much more? When Paddy Ashdown was Liberal Democrat leader he managed to oversee quite a few by-election victories and attracted some inward defectors. Under his leadership the party won by-elections at Eastbourne, Ribble Valley, Kincardine and Deeside, Newbury, Christchurch, Eastleigh and Littleborough and Saddleworth. The party also attracted two inward defectors who were sitting MPs – Emma Nicholson and Peter Thurnham, both from the Conservatives. In his diaries, Paddy Ashdown commented that an inward defector was the next best thing to a by-election victory.
So, the answer seems to be that a defector is worth somewhere between £900,000 and a by-election victory.