I think the margin is the wrong way to view this - yes, there are companies where losing a dollar will make it uneconomic - but I will be amazed if that company is otherwise a strong, scalable and robust business model, instead of a crippled business limping along till something kills it off.
Hiring decisions are made at the margin though. Even strong companies stop hiring when the additional cost of an additional worker is less than the additional value to the business.