350 HP 318 Build

When you're a one man operation, the billable hours only have to cover your time, materials and expenses. If you have more than one person operating in different capacities (sales, shipping, warranty, etc) now those same hours have to pay more peoples' paychecks. The rate has to go up to cover that.
That's not necessarily true, one man operation only has one to make a living off of and if he's got low overhead he may decide to charge less than the competition for an advantage but if he is already fairly busy he wouldn't need to and would be silly to if in high demand.

If talking a shop with multiple employees the employer generally only needs to make a smaller percentage off each since his income is based off many, Eg.. Walmart has like a 2 million employees so on average if they can make a net profit of $500 for every employee Walmart will have a billion in net profits and they had a net profit of $13.6 billion last year or about $ 6,800 average off each employee. Generally the more employees you got the less you need to make off each.