Prasad,
The color camera has the Bayer matrix in front of the pixels. It doesn't matter if you if you set the camera to "black and white mode" the light still has to pass through the color filters in front of each pixel. The image may be displayed in black and white artificially but the light passing through the red filter will only allow red light through. Only light of that wavelength will be recorded.
The camera may convert the signal to black and white after the RED light reaches the sensor, but the data correlates only to the red light that passed through the filter in the first place.
A mono sensor has no Bayer matrix so each pixel is capable of detecting all of the wavelengths since there are no filters in front of each pixel that only allow specific wavelengths through.
I hope I didn't confuse things more. As far as the cost is concerned it probably has to do with the manufacturing process vs overall demand but that is just a guess. Someone else may be able to provide a better answer.
What ever you decide do not try to scrape off/remove the Bayer matrix from a OSC, it will probably cause more problems than it would solve.
Clear skies,
-Lou
Canon EOS Rebel XS (full spectrum mod)
Astronomik CLS CCD
Orion thin off axis guider
QHY5L-II M guide camera
Celestron CGX,Edge HD 8,0.7x focal reducer
Optec TCF-Leo focuser