They vary by vehicle, I've seen them as cheap as $20 and as expensive as $150. Most fall in the middle somewhere.
Why do you want to replace them? The 2 downstream sensors are likely a different part number than the upstream sensors, also. The 2 upstream sensors are the only ones that would cause a real running issue.
Not to knock mechanics, but many of them don't understand how the system works. Often times it can even not be a faulty O2 sensor but something else making the sensor through a code.
You have a check engine light? What's the code?