I buy all of my cables and stuff from Monoprice. I don't think you can buy items of that quality muc cheaper elsewhere.
I have no idea how much thos mounts usually are, but those prices seem crazy inexpensive.
I've bought a ton of stuff from monoprice. Have done so for years. Cables, wiring, stands, cleaners, you name it.
For something that's gonna hold my TV though, I'm leery of going with one their products. Yeah, it might save me a couple hundy, but the consequences of the risk aren't worth it, IMO.