Very simple - Add a skill that gives a discount on upkeep.
5% per point and the max skill level is not dependent on either Strength or Agility it is simply maxed out at 10 points
So if i want i can have an 18/18 build with 10 points in "Repairs" which gives me a 50% discount on all my repair bills since i am doing it myself instead of going to the blacksmith
This would mean that people can afford to always use heavy gear/horses if they want, but they will have to sacrifice combat skills in order to do it.