Hi Jono
They usually come up on eBay for about £750. I was lucky enough to get one for £400 from someone who was breaking their car. You might want to also look into what caused the ECU to fail as, from my experience and from what I've read, there is an underlying cause. Mine was because my phonic wheels had completely rusted up meaning that the sensors were unable to read the wheel speed which fried the ECU.
Cheers
Tristan