There's also costs that scale with headcount but not experience. For instance, you've got to get computers and chairs and desks for everyone whether they're a rookie or an expert. Ditto things like healthcare. Sure, the rookies may have cubicles and slightly worse plans, but those costs aren't going to be 1/5 or 1/10 as expensive. And while the good programmers probably need less oversight than average, support personnel costs (internal IT, HR, recruitment, whatever) still scale with headcount.