A small clothing firm currently produces 50,000 shirts and blouses per month. The costs of its factory, raw materials, and labor is $500,000 per month. If the company is to increase production by 5,000 and that requires an additional labor and raw material expense of $100,000, what is the best estimate of costs of the increased production?

a. $100,000
b. $400,000
c. $500,000
d. $600,000