Adblue is a mixture of 32.5% synthetic urea and 67.5% de-ionised water - it varies greatly in price with small containers costing a fortune, 10/20 litre container costing about £1.20/litre but it's available on pump a about £0.50/litre at truck filling stations - beware the full flow rate is high as HGVs use a lot of it!
Consumption of Adblue will be between 1-6% of the amount of diesel consumed, dependent on engine load.