Price Line or Budget Line

Imagine you go shopping to buy notebooks and pens with only ₹40. Here, your budget puts a limit on what you can buy, creating your "price line" or budget line.

• Price Line/Budget Line shows all the combinations of two goods a consumer can purchase, given their total money income and market prices.​
• Ferguson: “The price line shows the combinations of goods that can be purchased if the entire money income is spent.”
• Prof. Hibdon: “The budget line shows all the different combinations of the two commodities that a consumer can purchase, given his money income and the price of the two commodities.”

• Draw a graph with “Food Units” on the X-axis and “Clothing Units” on the Y-axis.
• Plot the combinations from the table and connect with a straight line (budget line).
• The line slopes down because choosing more of one means less of the other, since money is limited.

Total Spending = (Food Units × Price of Food) (Clothing Units × Price of Clothing) = Budget

M = P_f  × Q_f + P_c × Q_c

• In this example: 20 × Q_f  + 40 × Q_c = 200

• Negatively sloped: Buying more of one means giving up the other
• Slope: \[\mathrm{Equals~}-\frac{P_f}{P_c}\], i.e., negative inverse of price ratio
• Straight line: Prices are fixed

• Change in Income: If income rises, the line shifts outward right in parallel, showing more buying power; if income falls, it shifts inward to the left.​
• Change in Price: If the price of X falls, the line rotates outward from the X-axis; if the price rises, it rotates inward. The same for Y with the Y-axis as a pivot.

• Equilibrium condition: Price line is tangent to the highest possible indifference curve (shows maximum satisfaction).
• Tangency condition: At equilibrium,

\[MRS_{xy}=\frac{\text{Price of X}}{\text{Price of Y}}\]

• Convexity condition: Indifference curve must be convex to the origin at the equilibrium point (shows preference for variety).

• The budget line shows maximum affordable combinations
• It has a negative slope and is straight when prices are fixed
• Shifts outward/inward with changes in income or prices
• Equilibrium occurs where budget line is tangent and indifference curve is convex