Commission

What are your margins? What is the average order? Is the salesperson only working for you? What other expenses are you covering. After commissions, are you both making money? Is the salesperson adding additional sales or part of the base sales (break even point)?

Then figure out what they can be expected to reasonable sell and how does that compare to other salespeople in your area (both inside and outside). Is it reasonable?

You'll probably have an beginning answer to your question.