I would say that the healing belt is not particularly broken as long as you only have one of them.
It is true that it costs less than comparable healing items from the core rules. The rejoinder that comparable healing items from the core rules are over-priced is right on target. Nobody bought wands of cure critical wounds. People would occasionally buy potions of cure serious wounds, but only because those potions were the only game in town when it came to in-combat healing. You couldn't get anything more powerful that was usable without the ability to cast spells. Of course, past level 5 or 6, it was inadequate and even before then it was too expensive to be used regularly.
The problem with the healing belt (and other x/day items from the MIC such as counterstrike bracers or the ring of thunderclaps) is that nothing other than shame at being so cheesy prevented you from making or buying multiples and just changing belts when you ran out of charges. That is the aspect of the belt that might possibly be broken (but you still need to consider the 10 belts of healing over the course of a long campaign before they become more efficient than ten wands of cure light wounds (or faith healing) or ten wands of lesser vigor and they put all of the costs of the effiency up front rather than spreading it out over the course of several levels as multiple wands would do). So, even if you deliberately abuse them, I'm not convinced that they're broken. (Other items like counterstrike bracers, on the other hand, are broken if they are abused in that manner).