Hi everyone and thanks for your responses to my question. I certainly get the message that a tiered commission rate would not be a useful incentive.
I'd like to comment a bit on the majority of the responses. First, any commission based on a percentage of the sale is fundamentally incentivized by the house's selling price. Otherwise, all commissions would be fixed-fee, unrelated to the selling price. While not a broker, I don't see much of a difference in the amount of work or outlay needed to sell a $250K house compared to a $500K house. Second, payments tied to performance above a certain benchmark level are very common in business-to-business transactions. For example, you might pay supplier more for delivering ahead of schedule, either by a lump sum bonus, or a higher item price (which more resembles a tiered commission). I don't really see why real estate seems so different than other sales transactions.
Anyway, thanks again everyone, and enjoy the holidays!