Cross-London Season tickets are manually created rather than being "off the shelf" like most tickets. They always include a Travelcard element giving you unlimited travel within Zones 1-6.
The calculation is to sum the appropriate Travelcards from origin and destination, and subtract the cost of a Zones 1-6 Travelcard - in this case 405.90+400.20-227.00=£579.10 per month. Season tickets can be purchased for any period between a month and a year, so that you can for example buy a ticket covering you until your next holiday and avoid paying for days you won't use.
As I don't drive I can't comment on the journey by car, but Google reckons on 1h15-2h which is similar to the train but probably more stressful owing to M25 and Reading traffic. OTOH on the train, crossing London is a pain and Thameslink is busy - the trains to/from Reading will be better though - busier in the evening but you can count on getting a seat. As suggested, changing at Farringdon makes it about as good as crossing London can get.
I guess that - if you do drive - it will come down to (a) how far from St Albans station you live, and where in Reading you will work, and (b) how much you value a journey by train over the extra expense, and possibly time (depending on (a)).