I can understand your frustration. It is strange to think about people at your wedding that you don't know.
I can tell you though, I was always the "single" girl and went to several destination weddings while single. Once I went alone and once I brought a guest. I had so much more fun at the wedding when I brought a guest. It's hard being single to travel alone, go to dinner alone (before/after the wedding day), and even feel comfortable at the wedding alone.
$200-$300 a person is a lot for each extra guest, so I understand your delima, but if your friend's presence is important to you, I would allow them to bring a guest, even if it's their sister, so they can have more fun and feel comfortable the entire weekend of your wedding.