If a jacket like that is likely to last at least 5 years, it seems like it would be worth it, even at $300. I am always leery of forking a few hundred bucks for something I'm unsure of, but hell, even my $50 Shimano shoes that I bought in 2011 are still perfectly functional. Now that I know they are that good, and last like they do, I'm much more inclined to pay more if needed (but sales are always welcome!).
I would probably get one of these jackets if 1) I still lived in Seattle, and 2) I didn't have a velomobile.