I love block erupter - I'll go to the post office tomorrow to get 8 of them.  However, I don't do my calculation that way you do.  I am going for the lottery style of mining.  That is, I am not going to join a pool.  But my erupter can get the lucky lottery ticket and win 25BTC in one second - maybe.  So I'll run it in the winter (I have electric heating so this makes it free to run) and when it hits I'll buy caviar!!  Whoo Hooo!