I've had both dirt cheap and good quality cogs.

I've heard expensive cogs make less noise. From high school physics I can guess that less noise is a result of less friction and slack in machining which would theoretically mean you have less energy lost in the drivetrain. Probably 0 possibility of measuring a performance change let alone feeling one when you're riding.

As far as holding up to wear goes, if your avenir lasted 2 years a fancy cog like the EAI gold medal ($89.00) would have to last over 25 years to have paid for itself in cheap cogs.

Bottom line: If money's burning a hole in your pocket and you've got nothing else to do, buy a fancy cog. If not, you'll probably be perfectly happy with another cheaper one.