When you're adding grade separations in an urban area, they have to be a "one-off." The "best" combination of features will be different at each site. Some paperwork streamlining is available, but ultimately each site needs special attention, and that costs money.

In rural areas, where land is cheaper and less interference from existing development exists, it should be possible to use a more or less standard design for several grade separations and cut cost a bit. Unfortunately, most rural counties and communities don't even have enough money for one, let alone several, so the potential scale economies are lost. They usually end up waiting for some big highway (or HSR?) project that adds grade separations as an incidental item.