In case anyone is interested Brownells is closing out their polymer lens RMSc for $279 which is a little more than the Holosun 507k but likely the cheapest these will be.
https://www.brownells.com/optics-mou...rod116190.aspx
I bought a few more because I like them on micro guns to not have to carry suppressor height BUIS.
So why pick this over a 507k?
It has one of the best auto adjust light sensors out there.
The Romeo Zero and 507k don’t have any auto light adjust.
It has a larger window and less obtrusive shroud. Sure it might get damaged easier in a drop but I’d rather have better window and light adjust than impact resistance for the kind of carrying I do.
YMMV
I think @GJM has used these in the past. It’s still the sight on my primary carry gun.