Everything hd said.
The Apollo is pretty darned sweet, no doubt. (1st choice)
You can also go with something like the IR Hunters that will work, but they also have magnification, so if you have much magnification on your scope as well, then you have a field of view that is TINY, but they can be used in that manner. They can be had for a few thousand.
Then there are the Eotechs and they start around, what, $16K and go up from there.
You have FLIRs, big name brand but not as amazing as they sound.
There are a lot of functional options out there (by 'lot' I mean more than 5, LOL), but you want value for the $ given your previous posts and the Apollo seems to fill that niche best.
If you check into the Apollo, make sure it comes with the aiming reticle so that it can be used as a stand alone sight. As Delta4-3 noted, Armasight stopped putting reticles in them for a while but have apparently added them back. So double check before buying.