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.