In D&D 5E, if you have a focus (like bards have instruments and clerics have holy symbols) for your spell casting do you need to pay for materials?
(PHB p.203, "Components") Your focus can only replace some of your material components. Specifically, components with a listed value are not substituted by the focus.
For example, if you wish to cast control water with its material component of "a drop of water and a pinch of dust" your focus will suffice. But if you wish to cast continual flame with its material component of "ruby dust worth 50gp, which the spell consumes" you need to provide that material component separately.
(It's worth noting that not all price-specified components are consumed during spellcasting: compare continual flame with something like Drawmij's instant summons. Priced components which are not consumed are still not substituted by a focus, though.)