Lidl have got these 25 litre Topmove rucksacks instore from next Thursday (05/06/14) priced at £8.99 which seems pretty reasonable, there are quite a few colours to choose from as well.
Product Information :-
Hard-wearing, tear-resistant outer material
Air-Dynamic Flow System for optimal air circulation and breathable padding
With a practical headphone opening
Includes protective rain cover
Capacity (L): 25