How do you evaluate the cost of hiring a new employee?
In terms of your tax exposure, you would need to consult with your tax preparer or advisor. A lot of factors can enter into the final tax obligation, depending on your particular circumstances.
Again, remember to include all costs, as that $40,000 per year employee will really cost you more like $50,000 to $60,000 depending on where you are and what kinds of benefits you are offering (or willing to offer).
That said, the idea of saving a wage over time can end up costing you a fortune in terms of leveraging your own time and effort in your own company.