You could be right. I just looked on pistonheads and can only get £4k.
But the money made from off loading the rest of the car still make the engine relatively cheap.
Exhaust, tyres, wheels, glass, dash, radio, seats, headlights, body panels, tail gate, door cards etc. etc. easily will net you over £1500.
http://www.pistonheads.com/classifieds/used-cars/toyota/supra/toyota-supra-jap-import-single-turbo-conversion-bargian/1609534
http://www.pistonheads.com/classifieds/used-cars/toyota/supra/toyota-supra-3-0-turbo-3dr-auto-ftsh--2-keys--vgc/1589009
I just can't be bothered with hassle of selling off the bits.