Replying to So how much do thule cargo boxes ruin gas mileage?

so i've heard contradicting stories so i'm resorting to NS to get some ideas. i was thinkin of throwin a thule box on my 03 subaru legacy sedan this winter. so here's my question for anyone who has one. please state your year & car model (not that it really matters i guess but i'm interested), and the MPG you lose by having a box on the top. and also if you'd recommend a box, or if it's a waste of money.

thanks