Independents incur the own expenses (Advertising, Rents, Photo Shoots, time answering Phones (particularly time wasters) cost of a second phone for business, security, consultants and regular checkups etc. and most of all their Discretion. Agencies does all that and MORE for the girls.
Advertising - its free on AW. Yes you pay a small amount to display your phone number on the days you work, but this is greatly offset by private gallery income.
Rents - They have rent to pay if they work independent or for the agency. The agencies usually make the girl work from her own house.
Photo shoots - rarely do WGs get professional photo shoots. Its usually ones taken with a mobile phonecam that are put on AW. And from topics here on the forum, these types of photos are generally preferred anyway. So no cost there too.
Cost of a second phone - they still need this to work via an agency. They'd be mad to use their regular phone when getting agency bookings as the agency will phone them throughout the day too. And its not really the cost of a second phone anyway. All they need is a second sim and all you need is pay as you go as you will be taking calls from people never making them. So no costs from phone calls. Plus a girl does not need a £600 Iphone. A £10 Pay as you go phone from Asda will and does suffice for taking calls
Consultants and regular checkups - all free at the Gum on the NHS.
Not really any extra costs that I can see from any of that.