Like Barry, I'm guessing that you're referring to the HP OfficeJet Pro 6970. If so, the inks for it are horrendously expensive! (Sorry!)
When comparing prices across different websites, check that you're doing so on a 'like for like' basis. HP's own inks come in three different fill sizes. (i.e. the actual cartridges are the same size but they amount of ink inside them varies).
CartridgePeople, for example, (which is a firm that's been recommended frequently by ABers) charge £46.56 for a full set of 'original' 8ml cartridges, £70.80 for 25ml ones and £99.99 for the 37ml type. (They're currently out of stock of their 'compatible' cartridges):
https://www.cartridgepeople.com/HP/OfficeJet-Pro/OfficeJet-Pro-6970-Ink-Cartridges
Inkredible, which is where I buy my inks from (as do many other satisfied ABers) charges £49.99 for a full set of 'compatible' 37ml cartridges, so that's half the price that you'd pay for 'originals' from CartridgePeople:
https://www.inkredible.co.uk/hp-officejet-pro-6970-ink-cartridges
Other suppliers might offer them marginally cheaper but I've been struggling to find any recommended supplier that currently has them in stock.