Oh, cavalry is an expensive class indeed. Forgot about you were asking an economical build.
I'd say hybrids would be a bit more expensive than specialists, as they tend to use more than 1 weapon type (more chance to break, and with less wpf even more chance to break). Ranged also has some extra repair costs, just because developers hate those classes. I'd say, go for a simplistic build. Something that will make you useful with economical equipment. Hmm Spearman (or pikeman) or offensive shielder are what coming to my mind.
As a spearman, wear medium armor and use a cost efficient spear to stop horses, stab people from behind etc. Bamboo spear, Long spear, war spear maybe.
As a shielder, wear light armor (maybe medium armor) and protect yourself with your shield. Get 5 shield skill minimum (and maybe optimum), grab a good 2 slot shield and... fight

Use your high movement as an advantage.
Edit: You can try agiwhore-katana bitch build too. 15-24 is a viable build for a katana. You won't be vulnerable to arrows at all. It is easy to dodge arrows, or find a cover quickly; as you will be reasonably fast. Wear ninja armor for style. Ninja armors are not very heavy, so not very expensive. You'll be making loads of money with this build.