The value of a 1990 Pontiac Firebird Trans Am can vary greatly depending on its condition, mileage, options, and history. Typically, you can expect to pay around $14,775 USD for a 1990 Pontiac Firebird Trans Am in good condition with average spec.

Many factors can increase a car's value including excellent condition, exceptional originality, a recent restoration, low mileage, unique provenance, desirable options, popular or unique color choices, and excellent maintenance records. Upgrades and modifications that enhance performance or aesthetics can also add value.

Factors that can decrease a car's value include unusually high mileage, poor maintenance history, visible wear and tear, accidents or damage, title issues, and gaps in ownership history. Additionally, less popular color choices and extensive modifications that don't appeal to a broad audience can reduce value.

Mileage plays a significant role in a classic car’s value. Lower mileage usually increases value because it indicates a higher degree of originality, is rarer, and suggests less wear and tear. However, cars with exceptionally low mileage may also have maintenance needs due to lack of use. Whether someone should pay more for a vehicle with lower miles will depend on how they plan on using it, and the vehicle's specific condition.

The value of modified cars differs widely based on the type and quality of modifications. While some modifications can significantly increase value, especially performance enhancements and high-quality custom work, others may not add much or could even detract from the car’s value if they appeal to only a limited audience.