When HP sell you a new printer (Deskjet 3050) they only give you starter cartridges with it which hold a very small amount of ink in them Hp 301 black 3ml and the Hp 301 colour 9ml cartridge. I was using about 3 black and 2 colour cartridge per month and it was costing me a small fortune. I later found out that Hp do higher yield versions of the 301 cartridges, which cost only a few pounds more but hold far more ink in them than the starter cartridge which I was using before. What you need is a HP 301XL on the black which hold 12mls of ink rather than the 3mls of ink that is in the 301 starter black cartridge and on the colour you should be using the HP 301XL colour cartridge which holds 11ml rather than the 9 that's in the 301 starter cartridge. I can even go one better, I now use a company called advantage online who stock the Hp 301XL originals plus do a British premium compatible 301XL bk and 301XL colour cartridge which hold 18mls black and 18mls colour. I have used them a couple of times so far and have been quite impressed. I hope you don't fall into the Hp trap of selling a very good printer (Deskjet 3050) at a very cheap price and then trying to charge you load of money for starter cartridges.
Its also a pity that Hp don't do a twin value pack on the XL's