The tactic of the cheaper miner:
Begin by creating a miner while building without doing any other unity.
When the ore is ready, wait for the money to be between $ 300 and $ 400 to begin construction of another miner (it will then be already built in half).
Then cancel the construction of a building already ready to not allow the money to be empty (otherwise the trick does not work).
Apparently it is necessary to abort the same unit as the unit that least expensive. The tactic runs intermittently, may quite often.
if someone have another tactic, he can give it here. any tactic is wellcome.