What does the Reduced engine power / limp mode mean?

The engine computer has limited power to protect the engine or transmission from a detected fault.

How much does it cost to fix the Reduced engine power / limp mode?

Typically $100 to $2,500 in the US, depending on the cause. A throttle body clean at the low end; a turbo or transmission repair at the high end.

Urgency:Service soon

Slowly, and only to a nearby shop. The car will feel very sluggish and may not exceed 30 to 40 mph. Avoid highways.

$100 – $2,500

A throttle body clean at the low end; a turbo or transmission repair at the high end.

A typical range, not a quote. Actual cost depends on the car, the shop and the fault.

Pull over, switch off for a minute and restart — it sometimes clears a transient fault.
Have the codes read; limp mode always stores a code.