Well...if you are asking my opinion...
I don't know the exact costs in producing them are, and I don't know if we are talking with undermounting structure in place (for a completely smooth finish), or dimples for exterior mounting (like the original).
Is it going to be something that needs finishing? or is it completely just paint and bolt on?
I did see a post here where the person said it cost $40-$50 and a $100 for the mold for him to have a local shop do it. And I did see MAS Racing Products used to sell some cheap copies for $60.
So for me I would give a range of $60 for low end needing work and doing my own mounting structure...to maybe $100 or maybe a bit higher...$120?...for something I can just toss on my Duster and go.
I know I can locally get a 1969 Camaro Z28 rear spoiler for $80, I have been looking at those to modify.
I have seen those on ebay and there are a lot in the $70 to $120 price range.
If it ended up at $150 or $200...i would just pass. Anything over $100 will make me think a little bit unless I know it's top quality.
I don't have the skills to make one from scratch, but I have done some fiberglass work that I probably could modify an exsisting spoiler if I needed to.
If someone can show me the costs then I am willing to pay them for their time and effort...i certainly don't expect them to work for free.
But on the otherhand I don't want to pay for something that has a huge markup...that's what Ebay is for...LOL
Paul