'Cost' is not really a matter of money here, rather a matter of materials and infrastructure - both things the GTA and PVN didn't have anymore after the 14 years war, after been f*cked up by the Shivans, after the destruction of Vasuda prime, the loss of Sol, the GTI rebellion and the HoL guerillas. It is explicitly stated in the Reconstruction entry that the Terran economy is ruined, and the same is to be expected to a lesser degree about the Vasudan one. They had probably millions if not billions of refugees to build new homes for, so they probably weren't able to build a single ship larger than a freighter during the 4-5 years following the end of the GW.