I think it is 10 years or 140 k miles in my book (2.0 HDI, 2011 MY)I've been running them for years and always changed the belt at 120k miles. Always done the water pump at the same time for the sake of an extra 30 - 40 quid.
Peugeot says they should be changed at 160k miles or 5 years.
Cost will be variable depending where you take it. Main dealer will always be more expensive, in the region of £400.
A timing belt kit is available. This includes tension rollers and all that.