Carnet - How low can you go?

[Niva Say Never]

Hi all,
It's getting into the final few months before my sister and i head off on our Cape to Cape trip.
It's almost time for me to start Carnet applications etc.
So.....does anyone have any idea how low I can realistically value our old 60 series landcruiser?

1989, 4L non turbo, 140,000 miles, superficially pretty rusty and the body work generally looks very rough (mechanically she's mint, and rock solid where it matters, but they're not to know that I guess)

£1000?
Or even lower....£800?

I really have no idea what the book price would be these days.
With Egypt on the Carnet, every £100 less on the value will make all the difference to us.

Any thoughts would be appreciated

Happy travels
Sam