What phrase is "you betcha" a descendant of?

"You bet you"? That's the closest I could think of. Or is it "you bet yourself", with the "self" omitted so it's quicker to say? Or is it something else altogether?

I find this undocumented but fairly compelling:

You betcha-- Contraction of "you bet your...." (life, ass, money, etc.). Also "you betcher", as in You betcha! or You betcher ass!

(from http://www.urbandictionary.com/define.php?term=You%20Betcha)

"You can bet your life on it."

This saying is hugely popular in Australia, where practically no-one speaks German or Nordic languages. We Aussies shorten everything, so "you betcha" is short (for us at least) for "you bet your life", and has been around for at least 50 years.