The great barracks allows you to build a second barracks in the same village, but the troops cost thrice the original amount.
The great barracks cannot be built in your capital.
|Training costs for Roman troops|
|Training costs for Teutonic troops|
|Training costs for Gallic troops|
The level dependent attributes (such as construction costs and generated culture points) can be found here.
For the full table of construction times click this link for server with 1x speed and this link for server with 3x speed.