Quite a few good options out there. There are 3 basic types of red dot sights. Standard tube red dots (Aimpoint mostly, comes in regular and micro sized varieties, Henzoldt RSA), Holo sights (EOTech) and reflex sights (Burris Fast Fire, Vortex Razor Red Dot). There are options that start as low as $50 (not recommended for anything but rim-fire) up to over $1000 for certain fancy EOTechs. Since you appear to be looking for something around the $500 mark, I'll focus on those.
For a little background, I've owned 3 Aimpoints (1 x M3, 1 x ML3, 1 x Micro H1), 1 EOTech (552) and a Vortex Razor Red Dot, so I feel I'm able to give some insight in to the issue. They're all well made and would, IMHO, work quite well on your XCR (I haven't tried the Vortex yet, TVPressPass did a good review of it).
I'll start by giving the pros and cons of each type, then suggest a few models that may work.
Reflex sights:
Pros:
Lightweight
Low profile
Good battery life
Cons:
Small FOV
dot can be blocked by debris
may require riser
sometimes use obscure battery
These are great to have as a back up to a magnified optic, on shotguns or on pistols. The biggest limitation is if using it alone on a tactical style rifle is you may need a riser to get a proper cheek weld. This adds weight and cost to the set up. Dots are usually between 3 and 8 MOA.
Trijicon RMR
$550
Vortex Razor Red Dot:
$530
Russian Cobra:
$400
Burris Fast Fire III:
$330
To give an idea of scale:
Holo Sights:
Pros:
Can have complex reticules
Some models use AA batteries
Readily available magnifiers
Wide FOV
Cons:
Heavy
Poor battery life (relatively, 600 hours or so)
Some models bulky
EOTechs, despite their imperfections, may be my favourite type of red dots. I really like the 65MOA circle with 1MOA dot reticule. You pay for this with battery life and a larger sight, but some models use AA batteries. Some guys in the states have used rechargeable AA batteries and hooked up a miniature solar panel to the sight, allowing for virtually unlimited use.
EOTech, various models:
XPS
512/552
$450-$650
Red Dots:
Full size:
Pros:
Unbelievable battery life (up to 60,000 hours)
Readily available magnifiers
decent FOV
Cons:
Heavy
Can be bulky (the reason I don't like the Aimpoint M4)
May need separate mount (i.e. Aimpoint M2/M3/Pro series)
There are many types of these, Aimpoint is definitely the most popular high end model, and is used by police and military all over the world. There are also Russian made collimating red dots that are quite good, but half the price of a Aimpoint. Reticules can vary from 2MOA to 6MOA.
Aimpoint M4:
$600
Aimpoint M3:
$450
Commie stuff:
$200
Micro Red Dot:
Pros:
Lightweight
Compact
Good battery life
Cons:
May need riser
narrow FOV
sometimes use obscure battery
Aimpoint Micro H1/T1:
$500-$600
Henzoldt RSA:
$475