Instead, you could try a larger commission of any sale price above X, where X is your low estimate of what your company is worth.
Edit: On the other hand, this tilts their incentive towards pursuing low-probability, high-payout situations. If someone is working with ten different companies, and they all compensate with this structure, getting one good sale and ignoring the other nine might be the most rewarding option. On the other hand, if there are nine companies with the "percentage of sale price" structure and one with "higher percentage of price-minus-threshold," compensation, the latter will get all the attention. So this boils down to a brutal, Darwinian struggle to pay your sales guy the right way. At some point, structuring compensation might waste more time and effort than just selling the company yourself.