Pistol Mounting Standard

[cycoblade]

Hey folks, so I'm just waiting on the transfer of a GP X-calibur O.R.
I checked the website and it says that it is 'RMR compatible'.
Well I checked the cost of an RMR and suffice it to say, I won't be getting one of those.

My question is, what other pistol red dots are RMR compatible and not worth more than a kidney?

Thanks for any and all constructive help you can offer.

 

[Gatehouse]

I use a Holosun 507 on my Shadow Systems 9mm. It's about $430