Tiby...Dell wont give their ink carts to any retailer forcing you to go through them online or ring India for replacements.

You have three options for cheaper alternatives;

1) Refill your existing cartridge. Saves the environment and you money.
2) JetTec make a compatible cartridge. It is good quality, made in the UK. Not cheap Chinese crap.
3) Those carts you pictured are low yield versions for your specific printer. the higher yield versions are an M4640 Black and an M4646 Colour. They have around twice the ink as the ones you have there but aren't twice the price.