I have 3 RX580 4GB and they have been mining at 29 mh/s for almost 2 months now. I didn't see a drop nor and increase.
GPU mining wont be profitable soon, save up and wait for FPGA/ASIC. They will be expensive.
ASIC D3 seller tell me that 150$/day but when i got it 2x$/day and will be 1x in next month...