Shall I buy 1 USB Block Erupter and after starting to gain some bucks, re-invest in another one and so on ?!
As long as you realise that, assuming free electrcity, it will take a year before the BE has "earned" enough to buy a 2nd one ...
This is harder to speculate on since the price will likely drop as well. Remember, they started at 2btc and have dropped 75% in 2-3 months.