When you buy a catcher's service, you pay 100. When you place them on the map and go for an expedition, you pay 100 as well.
You will continue to pay 100 each time you dispatch catchers on an expedition. I highly recommend not hiring more than 3 catchers by clicking "Hire one for 100" link. There is a "xNumber" on beside the type of catcher, this represents how many you can hire in an expedition. You can only send 3 catchers at a time, so it would be useless to have more than 3 (unless there will be a feature in the future that would kill a catcher in an expedition or you can add more catchers each run).
The 100 recurring balance you pay each time is for the expedition costs, not for hiring a new catcher.