Knock sensors

Well, Just bought a 1995 J30 about a month ago.
After a week, the car started working strange...rough idle, hesitation,...but only when the car is warmed up...when the engine is cold, no pb at all.
The seller sent me to his mechanic and it seems that it is the knock sensor (from ECM error code).
How much should I expect to pay to replace them?

Thanks for helping

Well, Just bought a 1995 J30 about a month ago.
After a week, the car started working strange...rough idle, hesitation,...but only when the car is warmed up...when the engine is cold, no pb at all.
The seller sent me to his mechanic and it seems that it is the knock sensor (from ECM error code).
How much should I expect to pay to replace them?

Thanks for helping

The knock sensor is $123 for Q45 , but it may be the same as J30.
It needs two for Q45.

http://www.infinitipartsusa.com/Product.asp?Productid=2322