I haven't tried drilling one, nor do I recommend it. I was just building on your idea to do so. If you can drill one hole, presumably you could drill more, or maybe just a larger one. I think the odds of success are low in any case, though.
$878 seems exorbitant, but maybe it is a special shape or type? Is it maybe laminated safety glass rather than dual pane (air space in-between)? Since it blocks a mirror, I'm guessing this is NOT a typical rectangular side window, which is the type that can be refurbished for $200-$250 each. However, if the shop quoted you replacement glass rather than re-sealing the existing glass, the price will be steep for dual pane glass. I just paid $411 for a 28x42" piece of tinted dual pane glass for my stick house [but that includes labor].
How hard is it to remove the window? Regular RV side windows are pretty simple, but the ones adjacent to the windshield are often more complex. You mentioned a co-pilot window - is it in the entry door?