This makes sense, you'd be staring at the monitor for hours a day but likely would spend the $500 on <something> and forget about it or more likely it would be direct deposited and just end up washing in with whatever monthly purchases you normally make.
Taxes are a non-trivial consideration. Google employees in California have an almost 50% marginal tax rate plus I imagine Google gets better prices than retail monitors (plus someone else maintains it for you).
So they'd need to give you ~+$1000 in salary/bonus instead of a $500 monitor to get the same monitor for personal use.
In order to have a marginal rate of 50% you would need to be making over $625k. In order to have a marginal rate of "almost" 50% you wuld need to be making over $312k.
And if you're getting paid that much by a GAMMA its because California's economy, and the laws that gave rise to that economy, give you the market power to demand that sort of a salary. Because you're sure as sh3t not getting paid that much to do the same job in Kansas.