Depends on when the difficulty changes, currently I've got one rig with 3 Ants and 4 Block erupters that can at times get me 0.001 per 24 Hours ( estimate 0.01 payout every 11-15 days currently ) so more hashing power, more money.  As I've bought 2 of the ants with what I earned in pools ( and a ton of ' dust ' from free BTC sites ) profit is relative.  If you are looking to get 1 BTC in a year or less, you really need around the max a Pi can handle ( 120 miners and related powered hubs ).