>>8437061First up, the mech in my second pic is from a cheapo line of candy toys called Animagear. But it probably wouldn't fit your purpose. It's a model kit for one thing. Not all are humanoid based. And they're actually SD-ish, which is why my mech doesn't have its head on, since it's an oversized Medabot style head.
As for recommendations, well, I suppose it kinda depends on a few factors. As you can see, I prefer the smaller 2 cm type of Army Men. But I assume you're leaning toward a more conventional 2 inch scale? And I assume you want a grittier, more generic looking style of mech?
Shipping aside, you can find a whole bunch of older Kaiyodo Revoltech for cheap. Guys like the Brocken or Soltic can be found for 6 to 10 bucks a pop. Bandai's Mobile Suit in Action line also has DIRT cheap mechs. Not just Gundam but those generic mechs from Code Geass. The Hammer Drones from Ironman II make excellent mechs too. None of these, shipping aside, should cost more than 10 bucks a pop. Some are like, just 3-6 bucks. They stand anywhere from 4 to 5.5 inches tall, which makes them good for my Army Men, not sure about yours. They'd still be nice mech armor if you're into bigger army men, I guess.
The bootleg Hardhead from Wei Jiang or whatever knock off company is also good. The quality is good for the price and he looks kinda generic as a robot too.