ROI = [(Payback - Investment)/Investment)]*100
investment is not only money... but its also time... if i spent 1 hour on a campaign that i got paid 100 and spent in terms of money 100... but spent 1 hour on it.. to me... thats worse then a 0% roi... thats negative... but i guess i learned that campaign sucked so... payback is more than 100...
but still this is retarded... its 0%