You really don't want to go spending your hard earned on Aquaone's media. It is just over priced slices of ordinary foam. You don't need the carbon trays unless you need to expel old medications or tannins from bogwood etc.
Like Supercoley1 mentioned, the pond dept. of the LFS is your cheapest source. I would, however, try to find flat foam rather than the eggboxing as you are limited to space under the spraybar. Even then, once you cut it up to fit in the chambers then the foam should not need replacing for literally years. Also from the pond dept. you can get a big bag of white wool media for first stage mechanical filtration (£2.99) which will last for ages. This can be cut to size then divided to the right thickness to fit under the spray bar and is cheap enough to replace at water change time.