Jumpstart and keyboard, trackpad works but power button does not

My Macbook Pro's (early 2008, 2.4 GHz model A1260 Core 2 Duo) power button is not responding at all. I managed to power it on by shorting the power pins on the motherboard. The keyboard, track pad, etc all are working fine and I am typing this question from the same laptop.

If it can be fixed inexpensively by changing just the power button and its associated circuitry, does anyone have a detailed instructions for that? Or do I need to change the entire top case?

I have cleaned out the dust inside when I opened the top case. This was not caused by any liquid spill. Was working fine till yesterday. I have gone through all the suggested fixes from SMC reset to using only AC or battery power, etc. but none has helped.