Every action RPG veteran knows not to bother buying items from shopkeepers. It's guaranteed that handing over money for a magic sword means two minutes later you'll find a better one lying on the ground. Your gold is better spent elsewhere in most action RPGs, but Diablo 4 in particular goes all-in on finding ways to part unwary adventurers from coins before the moment where you'll suddenly need it.
When you reach level 10 a couple of priority quests arrive in your quest log. One teaches you how to upgrade your healing potions, while another teaches you how to upgrade your items. And while forking over successively higher amounts of gold to your local Alchemist to increase the effect of every healing potion you quaff is totally worthwhile, the other is kind of a noob trap. Go ahead and do that one priority quest to get it out of your log, but there's no need to bother with item upgrading until you're much higher level.
Same goes for gem crafting, which appears as a priority quest at level 20. Remember when you'd just slap three chipped emeralds into a magic box and a flawed emerald would pop out? The Horadric Cube business was apparently privatized and by Diablo 3 we needed to pay some gold as well, a trend that has continued into Diablo 4 with Jeweler's fees now a significant potential money sink. If previous Diablos trained you to upgrade gems as a matter of course, you'll find yourself begging on the streets of Kyovashad in no time. Hold off on paying to upgrade those chipped skulls.
The worst noob trap, however, is the Occultist. The artisan who extracts aspects from legendary items and then imprints items with those aspects (or ones unlocked by clearing a dungeon for the first time, stored in your Codex of Power) charges thousands of gold for the service, plus a bunch of crafting materials. In the endgame you'll be making use of these services to craft legendary gear on the regular, but while leveling that won't be necessary. Even in World Tier 2, Diablo 4 isn't challenging enough to demand that kind of optimization.
Diablo 4 has plenty of ways to yoink gold from the pockets of unwary adventurers, with respec fees a particularly galling cost for those who'd like to experiment with their build. And if you're a gear hoarder, you'll be saving up 100,000 gold to upgrade your stash size ASAP. But it's the endgame aspect-shifting costs you really want to be putting gold aside for.
Fortunately, cash isn't that hard to come by. You can farm gold and XP by running dungeons, selling gear, and praying for a treasure goblin to cross your path. Still, it's better to finish the campaign with some money in the bank than have to frantically grind for it once it's necessary.