Tool costs are only really a problem in the early to mid-game. Basically a single flail can afford you 20 tools. Once you have some spare coin, try buy as many as you can carry if you see cheap tools. In the early game you can reduce repair cost by using shieldwall a lot. If you kill raiders with the flail head bashing skill you will get their fully repaired armor in most cases. Then just swap your damaged armor with the new one.
Generally, repairing Tier2+ weapons before you sell them will make you a ton of profit. Armors on the other hand use too many tools to make you any decent profit. If I recall correctly, armor starts getting profitable to repair at very high levels (reinforced mail hauberks, fallen hero armor).