It's your own fault for being so helpful.

I need to replace my wifes exhaust manifold. I have found pricing as low as $50 for a cast iron replacement, but I've also seen the OEM for more like $192. Why the large discrepency? Is there something I should stay away from? I bought a $35 dual core radiator off ebay last year and that hasn't failed yet, so I'm a little less shaky on buying aftermarket replacement parts, even if they aren't for performance. I just want to make sure I don't make a mistake and buy something that's going to blow up my engine.