You might want to check those prices. On their web site, those prices are for each speaker. I can't see the stores advertising and selling for half of the website. Just a thought.
I don't have any experience with metal roofs, but, live 30 miles away from Fm. A $20 powered tv-FM indoor antenna has allowed me to enjoy several stations that were very weak before. It is switchable to turn off when not needed. Hope this helps.