I do a few mines for my starting planet. Then I am back to trying to not go bankrupt, like many others.
By midgame, I am often making enough money to have a couple thousand around. I then put asteroid mines around high quality planets that will have more improvements to leverage the extra production. Planets surrounded by econ starbases are an early target as well. Any heavily specialized planet is a great target, especially if it has colonization bonuses. I limit distance to 6 tiles so I don't feel I am wasting anything. When the econ bases and wealth planets are kicking in, I will have lots of extra cash and will blanket asteroid mines almost as much as the AI does. I keep increasing my tolerance for the wastage involved in the Asteroid distance. The furthest radius I have used so far is 15, and that was just to throw excess money away. It doesn't really work, because the extra production ends up making extra wealth, but you gotta do something with all that coin.
You cannot look at Asteroid mines as an investment with a return. I don't think they ever really pay off, so to speak. You have to see them as a necessary expense to achieve maximum production, one of your primary goals. If you don't buy them, you will have less overall base production than your opponent, never a good thing. Production is key to so much. Every little bit counts. Asteroid mines are a necessary evil disguised as a necessary bonus. Very clever, if you ask me.