So in the place that i live, there is a very huge price difference between RX570 4g and RX580 8g.
considering that according to Whattomine the hashrate generated by these 2 cards only has a 10% difference, should i go ahead and buy 7 x 580 or should i buy 9 x 570 s?
Buying price is a one time cost. Well, yeah this is one of an essential factor.
Behind all of the things, you should consider how long you can stay at the crypto's mining industries. (
ignore the next step factors, let's start with the basics) U've known well about how much hashing speed will produce by the devices (as you wrote above), and also how much power will consume (even if it not, I think it's easy to find that info), right? So you were ready for efficiency measurement. This calculation can help you to decide which device is better for you, and it will show you which device is more efficient in short and long periods or some specific time (even with the different buying price).
Just remembering: Nowadays isn't a good moment for investing in crypto mining. But it doesn't mean U'll get nothing on mining.