I'd think that you'd want around 10 Ths (10 000 Ghs) to make it worthwhile. also you'd probably want to add a THs every month or so.
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I wouldn't even bother solo with this hashrate only 20+ if not more.
Right! And he'll only need 10,000 of those 2GHs Block Erupters to get to 20THs. Sounds like a great plan!