Origin of "Aviation Unit"

My wife Renee and I were having a conversation over dinner, along the lines of "Joe had this amazing mishap that's going to cost him at least a few aviation units". No, I'm not going to tell you who "Joe" was, though it was indeed pretty amazing. But I digress... Renee is wise to the ways of glider bums. She laughs hysterically when people ask "So Dave, how many gliders do you have?", and I don't even need to get my friends to register them to avoid discovery, as one acquaintance does. Only rule here is they can't be parked on the lawn. But I digress...

What is the origin of the "Aviation Unit" ? I've always believed it was to avoid wives finding out exactly how much we squander on our toys, though Renee suggests it may be to avoid realizing it ourselves as it might be truly painful. So, where did this start ? What's the earliest use of the term ?

Professor Cochrane, bonus points for explaining how an Aviation Unit has avoided inflation, when by all rights it should be up to $10k by now ?

Enquiring minds want to know.

It's winter...