I intellectually "value" Avios at 1-2p, but I'll generally not use them at a value of below 3p and probably average a good 5p. Of course, as a result I tend to have a million Avios in the bank.

It's possible to argue that's being irrational, but I never know if I'll keep up earnings rates (mine have slowed down by 80% in the last couple of years) and I don't want to be in a situation where trips become financially inviable for people because I've spent all my Avios (e.g. last minute trips to Asia, ideally in F class, peak season Nice flights, etc).