I like the 'cost of living' idea. Though for a world currency we would have to settle on one cost of living or have some sort of multiplier system.
Imagine 365 units == basic cost of living in South Africa. 1 unit == estimated amount of money needed to live 1 day.
Then you know you are being ripped off because beer costs 5 units in the us.
To take this further and give it a cynical twist, an iPad could cost "50 poverty days" given that most of the world lives off of less than 10$ a day. Yes sir, I would like a raise bringing my annual salary to 6000 poverty days.
There's also this:
https://en.wikipedia.org/wiki/Purchasing_power_parity.