In my research so far, I've identified three ways costs of an extension could be lowered.
1. Use of existing components and infrastructure, such as the buried footers. I've confirmed only one exists east of Future World.
2. Combining future extensions with larger future developments that overlap territory. For example, to the east of EPCOT, as well as NW (west of Living Seas and north of the EPCOT resorts), and directly south and center World Showcase are vacant land zoned for future development. Structures for these developments can be designed to support a monorail line running through them (ala Contemporary) and lowering overall costs.
3. Place the order whenever the LV monorail (which is based on the same specifications) places its order for the planned extension of that system (to the LV Airport). Placing the order for the parts with Bombardiar together in a single production run will lower the costs for both systems' extensions!