I've experienced both I think, Oil pump failure and a snapped cambelt
Oil pump failure is around £700 depending on where you go (inc pump, but not new belts or oil etc).
Snapped cambelt... More

I paid £700 for mine repaired in the end, but that included a free replacement complete head (bar the cams which were swapped over), and I managed to get all the belts and the manual tensioner conversion parts cheap from numberous sources on ebay. That damage can easily be over £1000 to repair depending on head damage (repair or replacement) and replacement valves and seals etc.