In Poundland today I picked up a 50 pack of 6x4 Polaroid branded 220gsm photo paper, and a 12 pack of A4 220gsm photo paper for £1 each.
This gives a page cost of 2p per 6x4 and around 8p per A4.
I tried a 6x4 in my Canon printer and it came out fine - better than Poundland's own brand 6x4 'Superior photo paper', which came out all smudgy and had no sharpness when I bought it a few months ago. Not quite as good as the Agfa branded photo paper I have, but that does have a higher gsm. Dries instantly too.
EDIT: 7x5 available too
"scotsann - Aug 30, 2010 20:12
Edinburgh St James' Centre also had 50 sheets of the 7x5. I got some on Friday."
Available in 7x5" matt or gloss finish. You can also get a pack of 3 sheets of A4 canvas paper for extra special photos.