Whatever the DM deems gold can be useful for. I allow the purchase of magic items. I use the guidelines for costs in the DMG and use loose negotiation. I enjoy this type of negotiation better than hard prices for magic item creation or purchase. It's fun to negotiate. I get to take time to think about what the person might need to make a magic item of a certain type. I can make up why it costs the price it does such as paying for a group of mercenaries to track down a particular component or paying a merchant to acquire the item from a far away place. If I make the price high enough, it motivates the player to pursue gold. At first my players had the same reaction about gold, but I've worked in a way to keep the gold useful. I even charge maintenance for fortresses if they want to keep their items. You can make gold as useful as you and your players find entertaining.