Someone else recommended this, and when I saw it I realized that I could just cut out some pieces of foamboard and use velcro tape to hold them to the sun visors in my Prius C. Works great.
Not with the actual 2018 Bolt part yet, but you can retrofit a late model telescopic Volt Visor: http://www.chevybolt.org/forum/226-interior/11922-telescopic-sun-visor-retrofit-working-vanity-lights.html#post141882
If you swing the visor around to the side so that it's parallel to the driver's door, you can then pull the visor toward the back of the car so that it covers the gap between the rear edge of the visor and the "B" pillar. It works great to block the glare when you're travelling at right angles to a sun that's low in the sky.
I'd be very, very surprised if there was any substantial cost difference. The biggest difference in my mind is that if you bought the Bolt visors then I'd expect that you wouldn't have to dremel off the extra plastic post on the mounting plate of the Volt visors. I assume that they haven't changed the mounting plate design between the 2017 and 2018 Bolts.
