Arriva bus have an on-line ordering shop, where you can purchase 4 week passes at a BIG reduced price compared to the weekly ON bus tarrif.
The monthly cost depends on your region and seems to vary up and down the uk.
But the local bus here is usually £12.50 p/week approx £50 per month.
So it's quite a saving buying the monthly pass online at £33.75.
Price also includes FREE p&p.
(Weekly tickets are due to go up in feb, so may be worth having a look)!
Link does work, try this one http://www.arrivabus.co.uk/multi-journey-saver-tickets/
For list of regions covered, click here http://www.arrivabus.co.uk/networkmaps.aspx