I'm assuming you're based in Uk? Are you including employers national insurance contributions and 5% compulsory employers pension contribution (from next year)? Are you allowing for statutory paid holiday of 12% of their earnings? Do you do pay for staff training? Have you calculated the cost of their wages whilst attending training and expressed this as a % of their gross?
Statutory holiday = 5.6 weeks per year, so 2.33 days a month at their normal earnings. It works out that you pay roughly 12% of pay in holiday, over and above what you pay them for the income they generate or their actual worked hours.
If you also provide training you should express the cost of that training as a percentage of their earnings. If you spend £500 in training for someone earning £20k a year, that's an investment in their future of 2.5% of their salary - more if they earn a fixed rate irrespective of their generated revenue. You both earn on that investment.
Sometimes you need to explain what slice of the gross revenue goes on staff costs. It's a lot more than you realise.