Wolf,
When a plumber comes to your house to fix your leaky sink, I know he can’t know what the problem is before he looks at it, but does he tell you his rate structure first? As long as he charges fairly, he doesn’t have to. Neither does a shadchan, unless she plans on charging more than the going rate. It’s also not a good idea to, because it makes her come across as greedy (I know it’s not logical, but that’s human nature).