You don'nt need as much army as you did in previous versions. So army cost more.
i dont agree. imperians units statistics are same as in v4a, but fortress are stronger than v4a. conclusion - we need bigger army than before and you have to remember that in v5 we can build fortress 9. in v4a there was a minimum amount 1000 units and here limit is 400 and we can gather resources from independent provinces which help to faster build army