It could be the shrub. Once a household gets the warehouse discount, the shrub goes a little wonky. Rather than selling it back and costing the house 1,000 or 10,000, it costs the house -1. That started with AL and the reputation fiasco.
While a large lavishly decorated home may cost 100,000 simoleans, if the money came from the shrub, the lot value (click the icon in the upper left while in Buy or Build) may be 0. This is more likely the cause of the low bills. Perhaps putting the shrub in a sim's inventory will bring the bills back to where they should be, but I don't know how that would effect the interest function. And then there's the forgetting they are their and the sim dies and the debt wiped clean...