Has anyone hired a commission-based sales rep for their business?

One way you could do it is give them a percentage of gross profits over a certain theshold - that way you make sure the jobs they are selling are profitable. For example: 

1) Gross Profit Bonus

• Measured quarterly using QuickBooks financials.
  - Tiers:
    * 50%–52% GP: 0.25% of Gross Profit
    * 52%–55% GP: 0.50% of Gross Profit
    * 55%–57% GP: 0.75% of Gross Profit
    * 57%+ GP: 1.00% of Gross Profit

This was for an ops manager position. Basically had a base salary then there is no cap on the amount they can make using these calculations. I don't know how sales reps would like that but it could be a hybrid model.