I just pulled numbers from some of the public transit buses that DoubleMap tracks. I see annual usage of 40k-50k miles on a single bus. Given 2013's diesel prices and standard diesel bus efficiency, that might be about $1/mile, so let's say $50k/yr in fuel for a single bus.

If they claim to save 20% on fuel, then a single bus would save about $10k/yr, and if the system can pay for itself in three years, then it would cost about $30k.

These mileage numbers aren't for London, so that estimate could be off by quite a bit. Still, consider than a standard diesel bus may cost $250k in the US while a hybrid 40' bus could cost $600k,[1] and this looks like a great deal.

[1] http://www.intercitytransit.com/SiteCollectionDocuments/hybr...