For the cost function example, the assumption is that cost increase between 1 and 2 units being produced is identical to the cost increase between 50 and 51 units, and the cost increase between 500 and 501 units.
Indeed the defining characteristic of a straight line from a modeling perspective, is that for every one unit change in x, y changes by a fixed amount regardless of the value of x. The change in y, is the slope of the line.
Do you believe this is likely?