Fixed cost: It mainly relates to time or period. It remains unchanged irrespective of volume of production like factory rent, insurance, etc. The cost per unit fluctuates according to the production. The cost per unit decreases if production increases and cost per unit increases if the production decreases. That is, the cost per unit is inversely proportional to the production. For example, if the factory rent is Rs 25,000 per month and the number of units produced in that month is 25,000, then the cost of rent per unit will be Rs 1 per unit. In case the production increases to 50,000 units, then the cost of rent per unit will be Rs 0.50 per unit.
Variable cost: Variable cost directly associates with unit. It increases or decreases according to the volume of production. Direct material and direct labor are the most common examples of variable cost. It means the variable cost per unit remains constant irrespective of production of units.
Semi-variable cost: A specific portion of these costs remains fixed and the balance portion is variable, depending on their use. For example, if the minimum electricity bill per month is Rs 5,000 for 1000 units and excess consumption, if any, is charged @ Rs 7.50 per unit. In this case, fixed electricity cost is Rs 5,000 and the total cost depends on the consumption of units in excess of 1000 units. Therefore, the cost per unit up to a certain level changes according to the volume of production, and after that, the cost per unit remains constant @ Rs 7.50 per unit.
A step cost is a cost that does not change steadily with changes in activity volume, but rather at discrete points. The concept is used when making investment decisions and deciding whether to accept additional customer orders. A step cost is a fixed cost within certain boundaries, outside of which it will change. When stated on a graph, step costs appear to be incurred in a stair step pattern, with no change over a certain volume range, then a sudden increase, then no change over the next (and higher) volume range, then another sudden increase, and so on. The same pattern applies in reverse when the volume of activity declines.
For example, a facility cost will remain steady until additional floor space is constructed, at which point the cost will increase to a new and higher level as the entity incurs new costs to maintain the additional floor space, to heat and air condition it, insure it, and so forth.