* Year and Model: Engines from different Mustang generations (e.g., 1965 vs. 1967 vs. 1970) will have different values. Even within a year, different engine options (e.g., 289, 302, 351, 428 Cobra Jet) will command different prices.
* Condition: A completely rebuilt, ready-to-install engine will be far more expensive than one needing a significant restoration. A "numbers-matching" engine (original to the car) will also command a premium.
* Completeness: Is it a complete, running engine, or just a short block (engine block, crankshaft, pistons, etc.)? The more complete it is, the higher the price.
* Modifications: Has the engine been modified? High-performance modifications can increase or decrease value depending on desirability.
* Source: Buying from a reputable seller or restoration shop will usually mean a higher price, but also greater assurance of quality. Buying from a private seller might be cheaper but carries more risk.
Therefore, you could find a usable, but needing work, engine for a few hundred dollars, while a fully restored, numbers-matching, high-performance engine could cost tens of thousands of dollars. There's no single answer. You need to specify the year, model, engine type, and condition to get a more accurate price range.
