what will be my monthly income if my device speed will be 1.5-2ghash/sec
$1.30 a month best case at today's price and difficulty.
The difficulty will go up, that's a given, which will lower the amount of BTC you'll earn over time.
The price, nobody knows exactly where the price is going.
https://bitcoinwisdom.com/bitcoin/difficulty1.5/2gh is a very very very small amount of mining power these days.