Looks nice, think they’ll last with the way it mounts?

They look like something Batman might throw, I like them.
Ride and report please.

How much and did they need any modification?

$20.87 Canadian delivered.
No modifications needed. They bolted right on.
If you look close you can see an adapter at the end of the mirror where it bolts to the cage. They come with this adapter. The ones on the mirror itself have what looks like a 3/8" thread and it goes into this adapter which has the metric thread that is the correct size.
You could modify these so that they don't stick out like a target, in case of a roll over, but you would have to make some kind of bracket. For twenty bucks I didn't think it was worth the hassle. Good enough.

Looks nice, think they’ll last with the way it mounts?

https://www.amazon.com/SupportTM-Universal-Motorcycle-Motorbike-Scooter/dp/B01GZZ9324?th=1

Forgot to add that the only thing that is plastic is the black part that the mirror itself is mounted in.
The rest of it is aluminum and metal.

How's the vibration factor? Do they stay in place on that round ball?

I don't know yet.
They seem to mount very sturdy.
You can tighten the round ball so tight that you can't even move that arm by hand. It's snug.
If anything will move it's the round ball in the mirror itself.

No one seems to have time to ride. Only been out twice this year.
We haven't been out on a ride since the original one got busted by a rock. Our summer was a joke with all those fires in the Province. It was weeks of smoke so dense you couldn't see the mountains from my window or sometimes even the end of my street.
And now the weather is bad around here -- lots of rain and I hear fresh snow on some of the mountain passes. So that means snow in some of the places we ride. It's also hunting season and haven't even had a chance to kill anything yet. Feel like a pro-lifer.

I hear ya about the weather.Seems like every year I complain about the rain here in Houston and this years no different.It's rained almost every day for a month straight now and still calling for rain out of the next 10 days,I'm sick sick sick of it.

Ride report:
Weather very nice here for nearly two weeks already, so we went for a ride up Hunter creek. It looks like there is to much vibration on an oddy for these mirrors. My buddy said you can see nothing out of them while riding but once stopped and waiting for others then ok LOL. They also move off position a lot.
Looks like I will have to do some mods on these.
I ran my brothers machine this time. Unfortunately I only had two batteries and three machines out but I thought I could just run it anyways. I was wrong. You need a battery to run the water pump. Ran two miles up the road before I noticed the water gauge not reading. Hope I didn't blow the good high perf Engine. It seems fine so far but I have to do leak down test before we take it out again. Damn, dummy.

Hope the engines alright!