All you have to do is hold the power button for 8 seconds, then tap it again.
Remember its not only the actual part cost, but there's also the associated inventory costs, added complexity for manufacturing which might include lenghier retooling (it's not just the 20c part, but a hole cut into each case, someone or a machine to plug the part into that hole, someone to wire the part to the motherboard, something extra to break for QC, something extra to break for warranty [which might mean $100 for postage since PC has to go back, be fixed and back to customer again]). In this way the removal of a part that costs 20c each can actually reduce costs in the long run by, let's say for example, an average of $1 per product!
The switch may only be costed at say $1, but thats $1 extra on the bottom line for every single PC sold. Putting it back wouldnt need $1 put on sale price to replace it - the manufacturer would add $1 to OEM price, or maybe $1 plus their markup of 15% = $1.15 since they wouldnt be having their ROI or returns ratios reduced. Pretending the retailer also has a standard markup of 15%, adding back the 20c part now has added £1.32 to the price of the product! Does it seem sensible for a highly competitive industry, that is greatly affected by turns in the economy, to keep this $1.32 price addition when another part of the product already serves it's function?
Plus, every other minor cost cut is another $x extra on the bottom line. Google for a study on airlines cutting a single olive from meals for a lenghy academic explanation.