Shortly: we hire the 20th worker worth 20 silver, cancel and get back 30 silver.

Details: when building a base, the first 20 workers cost 20 silver, and every next +10 workers cost 10 silver more. But if you start "training" the 20th (30th, 40th) worker, the game will think that you have already moved to the next level of the cost of workers and if you cancel the hiring of a worker, you will get 10 silver more than you spent. You can repeat the procedure as many times as you want.