how much is addon worth?

[pfkaok]

ok, so more than i should, i forget to take the addon when i'm multitabling. i always try to do it right when the break starts, but they don't let you until all tables are done, and so sometimes i just forget to hit it until the break is over.

i always get a little pissed, but i'm never sure just how much i'm costing myself when i don't take it. has anybody done some math on this to know just how much it costs for different stack sizes?