Travelcards start at £89 per month and go up to £245!! A bus pass costs only £53.80/month and can get you anywhere as long as you limit your travel options to bus and tram.
With Oyster, a single bus ticket, for ANY trip, costs £1. The cheapest Tube single costs £1.50 and they go up much higher.
So why is the Tube so much more expensive than buses? Okay, it might be faster, but IMHO the discomfort and cost of Tube travel far outweigh its advantages.
(Discomfort = 45 degrees in summer... tiny, cramped deep-level trains... endless flights of stairs, etc. Heck no, I won't pay a fortune for such torture.)