The difference between GDP at market prices and GDP at factor cost is

Gross Domestic Product at Market Prices (${\text{GDP}}_{\text{MP}}$) accounts for the market value of final products, which includes indirect product taxes and excludes government subsidies. The basic relationship is:
$${\text{GDP}}_{\text{MP}}={\text{GDP}}_{\text{FC}}+\text{Indirect Taxes}-\text{Subsidies}$$

Thus, the core tax factor differentiating production costs from retail market prices is the introduction of Indirect Taxes.