Why people buying ASICMINER shares @ 4.6? 95MH/s per share? When 100TH will come online it will hold 30% ofthe network. ASICMINER 10%. For some period of time 100TH will be most profitable. Even @ 0.9B/share it will deliver more profits than ASICMINER.

Now I understand - how people calculate profit.
Now 1 Ghs gives 0.48 btc/month. After 100Ths start to mine - difficulty will increase 1.5 times and monthly income on 1 Ghs will fall to 0.33 btc (0.066 btc/share).
With 100TH you are making the bet that the total dividends will be >= the amount you pay per share.
Yes, its simple. Just divide the price on monthly income. And monthly income will only fall every ~two weeks.
Anyone with half a brain has already calculated this. At current diff increase 100TH will still produce something like 600% ROI in 20 months. As long as they get at least 70TH online within a month.