Having done paid GM work, I am not a fan of it and I don't know why anyone would use such a service for a long term campaign that you would likely play online anyway. It is just too easy to find tables and players these day.$20 (USD) a month for a pre-published adventure is nuts. If you were doing your own prep, I'd argue that is reasonable, but $20 to read out of a book and make tweaks? No. If your going to charge, $10 for a published and $20 for an original is far more in line. Unless your $20 includes a semi-private provided space with it, big miss.
Anyway --
I reject the notion that running from a published adventure is easier than making up your own. I personally find it more difficult, because most published adventures are written and designed to be read, not played. Very few publishers do an actual good job of providing format that lets the GM "read and make tweaks." This is especially true of both WotC and Paizo, who know the majority of their customers never actually run the adventures they buy.