Thunderbrd
C2C War Dog
It's planned that they will be more expensive to maintain, though perhaps not more than many other units will be in their times. I see the problem in using them as open land recon and exploration due to how that competes with scouting units, but within the borders of other civs that's one of their primary roles and in many ways are the only ones that can sometimes. Defining the difference in the code would be an interesting trick.