It gets more expensive every day, so the trick would be to get rich quick, buy out the stock, and then keep buying out the re-stocks as quickly as they come in, up to a point where the cost can no longer be justified.
I don't recall how many Star Metals I've turned in, but in my current save workshop salary is 140, and on normal difficulty you get corruption only every other day, so based on that the max price for Blessed water would be 280.
At that price, buying one blessed water and then working in the workshop for two days has earned you a net benefit of +6 HP. Lower cost Blessed Water would mean you gain +6 HP and X amount of gold to be used for something else.
Now, obviously due to how the workshop works (you can basically only work every other day) and how Blessed Water re-stock and price increases work, you would use normally earned money early on to buy all the Blessed Water until the price hits the point you can no longer justify it, and work at the workshop once the pay is as high as you can reasonably get it.
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