Mines

Sorry, actually i didn't get the end of your sentence.

As you explained mines could introduced nice tactical option. But from my experience mines are almost never used in RTS, because the result is too uncertain regarding the investment.

If mines can be easily detected, it will be too easy to get rid off, so nobody will use them.
If mines can be detected by specific unit, the probability that your opponent will use mine, plus the cost of microing this detecting unit, will certainly result that nobody will use the detecting unit. Then it why i believe completely undetectable mines are simpler and better. And it is still possible to get rid of mines by sending canon fodder.

Widow mines are used a lot in SC2. Using banelings as mines is also widely used.

That's not a bad idea. It would force players to not send expensive units heads on and make the cannon fodder.
That means the mines should be more expensive than t1 units but much less expensive than t2 units. (between 90 and, let's say 200 metal depending on the power and special properties of the mine(s))
This way, it's cost effective for the attacker to let his doxes or ants blow up instead but cost ineffective if your t2 units get blown up.
That's a nice dynamic.

As well as Starcraft 1 Spider Mines which were kind of free.

Starcraft is the exception, as the map are corridor like and the game is based on microng unit, then mines are highly effective.

Concerning the price i think that mines should be even cheaper than T1 unit, because you have to take into account the price of the combat engineer and the microing required.

The argument 1 mine kill 1 DOX, then 1 mine should cost the same metal amount that 1 DOX is not fair. Do we have to pay the metal for the rounds because they destroy units ? when you use bomber you pay for the bomber not the bomb...

In Sins of A Solar Empire, mines were pretty cheap and covered a large area. They were usually not powerful enough to to take down units by themselves, but withered away enemy shields and armor so that the real defenses could deal with them easier.

Whenever you had ships in an enemy system, you could see were the mines were, but could not destroy them. For that you needed a scout to be in the system as well.

This made mines a powerful area denial tool as an enemy fleet could either take a bunch hits by going through the minefield, or go around and be under fire from the other defenses for longer.

Combat Fabricators seem like a good counter to mines. Let them see enemy mines in their FOV and allow them to reclaim. Or maybe a special order "diffuse" that is reclaiming, but limited to just mines.

It would be cool if the only way to destroy a mine was to reclaim it...

If it were invisible, that would be cool.

If advanced combat fabbers were somehow the only thing that could see mines, that would be okay. Just as long as it is limited to an otherwise unused unit. Might as well make them reclaimable-only as well.

Otherwise, invisible is the best option, and I am not saying it needs more expensive than a dox, if they were as cheap as walls its still cost effective to use a dox because of the cost of the combat fabber. Mines should be the cheapest way to kill units, but it really is kind of limited to how long a line of denial and how thickly layered, so they can never be overpowered based on best cost effective since they are situational, and clearing them with dox is an acceptable waste, losing a vanguard would just be hilarious for everyone except the sucker.