Cost

Paper: Principles of Microeconomics I
Semester: I / II
Study Material: Lesson 10 (Cost)

Introduction

This chapter explains the concept of cost in microeconomics and its relationship with production. After understanding how output is produced (Production theory), this unit examines how much it costs a firm to produce that output.

The analysis distinguishes between short-run and long-run cost structures. It explains different types of costs, cost curves, their relationships, and how they are derived from the production function. Cost theory forms the foundation for later analysis of firm equilibrium and market structure.

Chapter Overview Table

Official Scope of Chapter

The chapter includes:

• Meaning and classification of cost
• Money cost vs real cost
• Explicit and implicit cost
• Fixed and variable cost
• Total, average and marginal cost
• Short-run cost curves
• Long-run cost curves
• Relationship between cost curves
• Economies and diseconomies of scale
• Derivation of cost from production function

Concept Explanation

1. Meaning of Cost

Cost refers to the expenditure incurred by a firm to produce goods or services.

In economics, cost includes:

• Actual payments made (explicit costs)
• Opportunity cost of owned resources (implicit costs)

2. Types of Costs

(A) Explicit and Implicit Costs

Economic cost = Explicit cost + Implicit cost

(B) Fixed and Variable Costs (Short Run)

Total Cost (TC) = FC + VC

3. Short-Run Cost Curves

(i) Total Cost (TC)

• TC curve is upward sloping.
• Initially increases at decreasing rate.
• Later increases at increasing rate.
• Shape depends on Law of Diminishing Returns.

(ii) Average Cost (AC)

[ AC = \frac{TC}{Q} ]

Also called Average Total Cost (ATC).

(iii) Average Fixed Cost (AFC)

[ AFC = \frac{FC}{Q} ]

• Always downward sloping.
• Rectangular hyperbola shape.

(iv) Average Variable Cost (AVC)

[ AVC = \frac{VC}{Q} ]

• U-shaped curve.

(v) Marginal Cost (MC)

[ MC = \frac{ΔTC}{ΔQ} ]

• U-shaped curve.
• Derived from diminishing returns.

Relationship Between Cost Curves

1. MC cuts AVC and AC at their minimum points.
2. When MC < AC → AC falls.
3. When MC > AC → AC rises.
4. AFC continuously falls.
5. AC = AVC + AFC.

Why Are Cost Curves U-Shaped?

In short run:

• Initially: Better utilization of fixed factors → Falling cost.
• Later: Law of diminishing returns operates → Rising cost.

Thus:

• AVC and AC are U-shaped.
• MC is also U-shaped.

Long-Run Cost

In long run:

• All factors are variable.
• No fixed cost.
• Firm can change plant size.

Long-Run Average Cost (LAC)

• Also called Envelope Curve.
• Derived from short-run average cost curves.
• U-shaped due to economies and diseconomies of scale.

Economies and Diseconomies of Scale

Economies of Scale (LAC falling)

• Technical specialization
• Managerial efficiency
• Bulk buying
• Better technology

Diseconomies of Scale (LAC rising)

• Coordination difficulties
• Managerial inefficiency
• Communication gaps

Derivation of Cost from Production Function

Cost is inversely related to productivity:

• Higher productivity → Lower cost
• Diminishing returns → Rising marginal cost

When MP rises → MC falls
When MP falls → MC rises

Thus, cost curves are derived from production behavior.

Important Methods / Formats

Total Cost:

[ TC = TFC + TVC ]

Average Cost:

[ AC = \frac{TC}{Q} ]

Marginal Cost:

[ MC = \frac{ΔTC}{ΔQ} ]

Relationship:

[ MC = \frac{W}{MP_L} ]
(Where W = wage rate, MPL = marginal product of labour)

Exam Pattern Insight

Delhi University does not publish fixed unit-wise weightage. The following analysis is based on previous year question papers.

Cost theory is frequently asked in long-answer format with diagrams.

Practical Weightage Insight

Frequently Asked Areas

• Relationship between AC and MC
• Shape of short-run cost curves
• Economies and diseconomies of scale
• Difference between short-run and long-run cost

Typical Marks Range

• 5–8 marks: Definitions and differences
• 10–12 marks: AC–MC relationship with diagram
• 15 marks: Long-run cost curve explanation

Nature of Questions

Mostly theoretical with diagram explanation.

Common Mistakes Students Make

• Confusing AFC and AVC
• Not explaining why curves are U-shaped
• Forgetting to show MC cutting AC at minimum
• Mixing short-run and long-run cost logic

Common Areas of Confusion

• Why MC determines shape of AC
• Why LAC is called envelope curve
• Difference between economies of scale and returns to scale

Typical Question Formats

• Define and explain different types of cost.
• Explain the relationship between AC and MC with diagram.
• Why are short-run cost curves U-shaped?
• Explain long-run average cost curve.
• Distinguish between economies and diseconomies of scale.

Important Subtopics to Prioritize

1. AC–MC relationship (very important)
2. Short-run cost curves (diagram compulsory)
3. Long-run average cost (envelope concept)
4. Economies and diseconomies of scale
5. Fixed vs Variable cost distinction

Related Chapters

• Production (Lesson 9)
• Economics of Scale (Lesson 11)
• Perfect Competition (Lesson 12)

This chapter connects production theory with firm behavior. Clear understanding of cost curves is essential before studying revenue and market equilibrium.

Also Read:

Production DU SOL B.Com (Hons) | Principles of Microeconomics | Semester 1

DU SOL B.Com (Hons) Principles of Microeconomics Syllabus – Semester 1