without diagnostics, it is impossible to say what's wrong- much less give you a price!
You want to hook it up to a diagnostic computer and look at fuel trims (long and short term) as well as air flows, temperatures (intake, coolant, oil, etc) timing afdvance, etc.
If a sensor is bad and the car is running rich, you will see the DME trying to compensate (and failing, but not enough to trip a CEL)...this will point you to what the issue may be. A car with a sluggish VANOS; a car that is running cool and is stuck in 'open loop' control...all possible causes.
No warranty?
A