Don't get me wrong, I like Mining robots. They are one of the lowest risk ways of getting sensible amounts of money in the game. I think they are a little too good, though.
One 'improved mining robot' will pay itself off, with profit, after one use. And you can keep using them for practically ever with very little* risk.
* Protip: Stations at the edge of the map tend to be safer than ones near the middle. The middle of the map usually turns into a war zone.
I would suggest increasing the price a lot. Maybe by four times or so. Or mining robots could be 'place once' (can't be moved from initial position) and eventually that area runs out of resources. Or maybe mining robots could be heavy enough that they reduce cargo slots by one. Just some thoughts.