How much should i expect to pay for a replacement pressure sensor?

i have a Glowworm flexicom 30xc and the F9 code keeps coming on and now it wont ignite at all. i have tried opening up the valves underneath to raise the pressure (which has worked before) but nothing happens. The suggestion from other answers on this forum is that it is a faulty pressure sensor. I have an engineer coming out next week. How much should i expect to pay for a replacement sensor?