If they really don't do any advertising, it's neither an agency nor a brothel, just an incall venue with facilities that you can rent. Charging you for outcalls to clients that you've found yourself makes no sense.

In Britain, fees are like this: For incalls in a venue provided by the house, they take up to 50%, unless you're already paying rental or a flat house fee. For outcalls or incalls in your own premises, an agency typically takes 30%, though it can be less or more. I'd need a good reason to pay more. Things might be different where you are though, so it would be good to find a local site where you can ask how things work.