Your all-in cost [+] is generally modeled as about 140%~160% of base, assuming that level of base compensation. You don't have meaningful returns to scale between 1 and 3 -- "headcount" (gah, I hate that word) is a word largely because it has similar marginal costs on your 2nd and 2,000th employees.

[ + ] Health insurance, taxes, 401-k benefits, unemployment insurance / worker's comp, etc.

Same answer as @patio11 here. Usually you can plan on about 150-160% of base per employee to cover taxes and a standard set of benefits (i.e. health insurance, 401k). These numbers typically do not decrease at scale. Although with health benefits you may be able to negotiate a slightly better rate per employee at scale. But employment taxes, etc.. are always the same with one employee or 10,000.

If you start getting into exotic benefits (i.e. free meals, transportation, dry cleaning, etc...) the costs could be higher. However, these type of benefits will usually decrease with more employees as you may be able to get a better rate with more employees on the plan.

From my own research it depends greatly on the benefits you want to provide. Do you want them to have a deductible? How much? Should they have a copay? What about prescriptions, dental, eyes, and out of network? That said from my own research $1000 a month per person would be pretty nice coverage. Many companies cover a percentage of the payment, say 50-80 percent.