Where does the rushing units cost come from, please could someone explain this to me?

And if someone does not understand what i mean, i mean that if you for example rush an barracks with 5 turns left and produce 2 hammers, where does the cost be calculated from?

The cost is worked out by multiplying the number of hammers left to produce the building/unit, multiplied by a factor. The factor increases as you move from the ancient age to the modern age.

I don't recall the exact values of the factor for each age, but I'm sure I've seen them posted before somewhere so hopefully someone will chime in with them.