I think if you feel you cannot make any money in every month then you don't have to buy a rig because for reaching ROI, you still need to wait several months and its dependence on the price of the coin that you're mining too. but if you want to mining and not going to sell the coin and only sell for a long time then you can try to mine and you should buy a rig. there are too many things that you need to handle and you need to know how the procedure works so you can leave the rig mining for you. besides that, the electricity fee is the most important that you should know and calculate because this is the most expenses that you need to pay in every month.