Great question! I calibrated the influence costs by looking at treasure drops and typical economy (using weapons and armor costs as well as Keep costs if the system published that.) between the games.
The idea with non-diagetically acquired influence is that the purchase represents the physical aspect, maintenance, employees, etc. In the case of the horse and cart that also includes a driver or two and a place to park it when off duty.
I set the cost at 1 since that was the lowest integer for a less complex influence. Do you think the 2000gp Influence cost is fair for 5E? My experience has been that treasure flows pretty freely in 5E, but I am open to having over-estimated that!