If Antminer a9 Antminer S9 generates about 0.3 bitcoins per month, which is about 4000 us$, why people talks about 300us$ incoming per month??
Who told you an S9 could generate 0.3 Bitcoins per month?
They lied. You'd be lucky to see 0.06 per month and you'd have a utility bill of ~$100 (if $0.10/kWh for electricity).
That's providing the difficulty stays were it is for a couple of adjustment periods... which doesn't happen very often.
Thank you for your clarification
I saw it several places, some articles in internet and even a video talking about best hardware allows you to make about 3000 us$ per month, but electricity bill will be about 3000 us$ too... so, the information on the net is so weird
In this article for example:
https://www.buybitcoinworldwide.com/mining/hardware/antminer-s9/Author says:
According to the above inputs, the S9 will produce** 0.285 BTC / $159 per month** and 3.36 BTC / $1939 per year.
But 0.285 BTC is much more than 159us$... this motivates me to ask here