I was going to post some of the lamentations of a horse archer last week but respec'd back to melee instead. So here's my data from a ~31,857 gold setup as a horse archer (Light armor, Rouncey, Arabian Cav sword, Yumi, 1 set Tatar, 1 set regular arrows.)
Gold ------------ XP
528,679 = [did not note XP total] (Bought appropriate gear and respec'd into horse archer.)
525,958 = 1 891 680
527,391 = 1 979 040
525,832 = 2 065 280
524,062 = 2 170 560
520,452 = 2 226 560 (Upset with losing money. Exchanged Arabian Cav Sword for one of the cheaper 1H swords.)
518,624 = 2 243 360
As others have pointed out, high PD and ~50 1H proficiency increased repairs significantly but even if you double arrow upkeep I was still well under the "sustainable" figures posted by many people in this thread. I don't know about game/item balance but it is very obvious that playing a moderately-geared horse archer is unsustainable without consistent valor.