Just been to Aldi, where I bought packs of 10 cardboard boxes for £1.99.
Each box is 60cm x 36cm x 39cm. Works out at 20p per box, and I think thats a good price if you're buying boxes. Obviously cheaper to get free used boxes!
I bought 50 boxes for a tenner - from the Kingsbury store, not sure if it's national but mught be worth a look.
They were marked "when the're gone the're gone"
And for those that are interested, these are really very good, double walled corrugated boxes, with reinforced hand holes. Designed in a way that you don't need to tape the box closed, it folds into itself, so it's easy to build, close, re-open and store/re-use when your done.
I didn't know all that when I bought them, so I also bought 5 x packing tapes for 2.99 and a tape gun for 2.99, not sure if that's cheap but I think the're not bad either.