Standard class monthly season for St Albans City to London Thameslink* is £238.10 (will go up in the January fare increase, however).
* valid to St Pancras/Farringdon/City Thameslink/Blackfriars/Elephant & Castle/London Bridge
As you're travelling across London then there wouldn't really be any alternatives, I'm afraid as you'd need to have a cross London transfer included in your ticket which you can't have on seasons except as an All-Zones Travelcard (which will cost considerably more).