I'll play devil's advocate here. If we were literally talking about a simple piece of plastic, I'd agree with you.

I would guess that difference in raw materials cost of plastic versus metal is not significant enough to make a big impact on the final price that we see at the LBS/online. I'd say $4-5 at most, and that's a huge stretch IMO. Also consider that most "plastic" fenders are actually sandwich composites with other materials. PB is an exception, and is a possible reason that the PB's cost less than SKS.

Regardless, fenders of any quality have been engineered, designed, tested (R&D), tooled for manufacture, yadda yadda yadda, before they ever become a fender.
Add to that the extensive use of stainless steel hardware, stays, etc, plus overhead, shipping, and then the not insignificant LBS markup.

All of that considered, $40-$50 seems reasonable value to me, partucularly from the quality that I have seen in this price range (SKS, etc).
If they wanted $100 or more, then I'd be prone to agree with you.
I have used 5-6 different fenders from different manufacturers, metal and plastic, and can see merits to using either, and frankly feel they offer similar performance and quality for a given price point.