If you buy 2 TH in S3 is ~2000 bucks.
2000/ 478 = 4.18 *.74 = 3.09 * 650 = $2012
There is no reason to pay more than that today. Looks like we are solidly at $1 per GH/s
Batch 3 S3 are only 0.725 btc each. Also the S3 can be overclocked to at least 550gh.