You guys say 100k gross a year....
Our employees make roughly 40-50,000 a year. Health/dental/insurance/ei/other benefits costs probably 10-15k per employee. So that's 50-65k net. Material cost... Other labor...machine ware and tear, etc... You're making almost nothing from said employee.
That said, there's other factors. The most important one is if it's worth it to you.
If this new employee will cut back the amount of time you're working and allow you to rest if you need it... Or free you up to focus on getting more work if that's what you want, even if you break even on the employee it's worth it.
Every shop is different, wages are different... Net vs gross is different... If a shop is netting 50% on 100,000 they can easily afford a new employee.... If a shop is netting $20,000 on 200,000 obviously they can't.
Look at your finances and ask yourself if you can afford to hire someone, and whether you'd rather the cash, or the free time.
I remember a thread back when I first started asking everyone if they regret going from a small 1-2 man shop to a large shop... Most people said yes, that the workload quadrupled while the profit increased only slightly due to all the employees they had to hire. Bigger isn't always better... If you're still taking in low paying jobs, you may find it cheaper to stop doing those than it'd be to hire someone else.