The simple answer is there is no hard and fast value.

The lowest value is around 0.6p redeemed for hotel bookings.
Avios can be purchased for 1.66p (24,000 for £399 normally).

http://www.headforpoints.com/2012/12...irmiles-worth/

Some of the calculations above I do not quite agree with (sorry Raffles!), as it ignores the loss of avios/TPs earnt on cash fares. It also ignores the flexibility provided by avios bookings, which is harder to quantify.