They should definitely be cheaper, yes. But that's hardly enough, is it? They can fool the AI but any human player will simply phase jump to a clear spot. Heck, you can even just send a couple of scouts ahead and have them detect, leaving them useless. Their greatest shortcoming is really the fact that they simply aren't very effective, not their outrageously high cost.
I don't think the mopping up would be a problem were they invisible. The detection radius would clearly have to be made much bigger, probably to be given a range akin of weapon range of a LRF if not Starfish/Ogrev. That would also make the whole detection process a lot less cumbersome - which currently, it certainly is. Moreover, taking over the planet (colonizing) should reveal (and disable?) all remaining mines so we can skip the entire silly micromanagement mop-up phase and get on with what the game is/should be about: Strategy.
Regardless, if(?) mines can't be made invisible we should think up another solution. I don't think simply lowering the cost would be enough. Even if dirt cheap, they would still be more of a nuissance than a real threat. A minefield should be a threat - not to mention a cool as heck game feature! How would you like to be able to defend yourself with a huge minefield rather than always the same-old starbase? It's a whole new dimension to defending planets. Apparently, mines were supposed to be one of the major selling points of Entrenchment, a feature widely asked for. They really need to be made useful and not forgotten!
From the Entrenchment readme:
The most requested defensive feature was minefields and Entrenchment delivers.
Delivers? Hardly!
I'd say up the damage by a factor 3 or 4 (even a siege frigate runs through these things - what kind of mine is that?!), increase their range (currently units have to be almost colliding into them) and drastically lower the cost.
So really, what I'm asking for are minefields that are a lot more dangerous and can play an actual role in a defense while also being much less tedious to micromanage, both when deploying them and cleaning up afterwards. Sounds like a win, no?