Vavle is evil, but Darwinia is made by a small independent company who deserves your money. Is it "worth" the asking price? Well, to be honest, I don't think so - I think it would be more "appropriately" priced at $15-20. Is it worth getting though? Yes. And is it worth paying the asking price instead of the price it's actually "worth" in order to help out a small, independent game maker? Abso-fucking-lutely.
Not to mention, it's a really fun little game
I also hear there are lots of mods you can get for it which make it even better, but I haven't tried any of that out yet.