For U.K. FP fanatics the Tesco "Ink Cartridge Pen" is a very reasonable option at £2. I used these when I was younger before I bought my LAMYs. I would thoroughly recommend
TRF | Tesco Clubcard
Remanufactured Ink cartridge Tesco HP H70 Black | Electronics \ Peripherals \ Printers \ Ink cartridges | Dropmax
Tesco Remanufactured HP 301 Multipack Ink Cartridges - Compare Prices & Where To Buy - Trolley.co.uk