Guys tell me tell me that I made the correct choice,I just bought 68 refurbished avalonminer 741 with 1 year warranty for 14000 euros,thats 510 th, so I paid 25 euros per TH,if I had gone with the S9i new with 3 months warranty,I would have bought with 14000 14 miners with a total of 196 TH which is 72 euro per TH.So I already have 110TH in the pool and I will reach 620 by the end of june,I hope i made the right choice.
Long term, the electricity cost will decide how long you can keep the the miners running profitably.
But when the hash rate you start with is so much higher for the same price, I'd say you are looking good

If Diff averages 8% for the next 100 days, you should mine around 2BTC in 100 days
If Diff averages 4% for the next 100 days, you should mine around 2.27BTC in 100 days
http://tradebtc.net/bitcalc.php