Your right the button is a silly idea. An ankle bracelet tethered to your desk is the way to go. As far as cost the amount you get offered is based upon the cost of the area you live in or at least was. Why should the employer not get any of the gains made from working at home?
Also a camera so you have to show up in a full a suit and perhaps a breathalyzer you can be asked to blow into now and then.