They have the Canon PGI-5BK listed for $5 each while the next cheapest was $17.90. The small catch is that these cartridges don't come with a chip which means that you need to keep the one from your current cartridge. You will receive instructions with these cartridges explaining how you attach the new chip.
Which in my way of thinking is a small inconvenience for saving $12.90 a cartridge.
Consumtech have a really good deal on shipping too - $10 for under under 50 items and free for over 50.
I have never done this with my ink cartridges and it would be great to hear from someone that has any experience in this field