There are two types of production, base production and bonus production.

Base production is provided by your Initial Colony, factories and research labs, asteroid mines, manufacturing/research bonus-tiles, and every other building that simply adds an mp or rp value. Each hammer or flask produced by base production costs 1bc.

Bonus production is provided by everything that adds a percentage bonus to your production. Fusion Power Plants, Research Coordinator, Moon/Rings in orbit of the planet, bonuses granted by (colonisation) events, Economic Starbases, your Social/Military Production and Research abilities, etc. You only pay half for the bonus production. The rest is free. In the Finance Managements screen, the amount you pay is shown under the label Bonus Prod/Research.

In your example, the additional hammer is bonus production. Probably either from a Moon or from your Social Production ability. Because it is only one additional hammer, it is free (1 / 2 = 0.5 = 0). The game rounds down after almost all calculations.