Getting Rupees in The Legend of Zelda: Tears of the Kingdom isn't as simple as one might think. It isn't a very common treasure chest item, and doesn't usually appear as a dropped item when you smash a ton of pots like in previous Zelda titles.
Most of the time, you won't directly receive Rupees. Instead, you'll often find yourself selling items to traveling salespeople, which is where you will make your most revenue. Here are a few ways to rack up on high-value items.
Enemy Boss Battles
The first method for stocking up on expensive items is defeating boss enemies across Hyrule. This will include the Battle Talus, Hinox, Boss Bokoblins, and much more. You'll come across these enemies all over the map; all you'll need to do is do a little bit of traversing, and you're bound to find an enemy Bokoblin camp or a boss waiting by a bridge.
To sell the items you pick up from these enemies, you'll need to find a seller willing to buy them. There are a few different options available to you, including traveling salespeople who you'll find wandering the roads on horses, Beedle at horse stables, as well as local sellers within towns. A great early-game seller can be found within Lookout Landing in Central Hyrule named Mubs. You can sell the enemy items to her, with some being worth more than 20 Rupees a piece.
Caves
The best way to earn a ton of Rupees from a single item is by scouring for gems within caves. In here, you can find Amber, Opal, Flint, Rock Salt, Ruby, and more. A few caves you can visit in the sky include the Mining Cave, Pondside Cave, Bottomless Cave, and Pit Cave. These will all have an abundance of gems, especially Amber which can fetch for a decent price. On the surface, you can visit the Ancient Tree Stump Cave in Hyrule Field, Sahasra Slope Cave in West Necluda, Mapla Point Cave in East Necluda, and much more.
Again, you can sell these items to any traveling sellers you come across. Opal will go for 30 Rupees a piece, Amber for 10 Rupees, and Ruby will go for a whopping 110 Rupees. Rock Salt and Flint won't go for much, but they can be used for other things like cooking, so it's best to keep them anyway.
Recipes
Another unexpected way to make a decent amount of cash quickly is by selling your cooked recipes. Of course, you'll want to keep a lot of the meals you cook, so you can survive out in the wild, but if you find yourself with a lot of extra ingredients, then whip together a decent meal to make a profit!
For example, dishes like Meat Skewers can go for up to 33 Rupees a piece, and some recipes can even get up to 48 Rupees, like Hasty Elixir. When you are still towards the beginning of the game, you might not have much on you except ingredients, so this is a great method to make a bit of Rupees at the start. Remember, don't sell dishes that are going to be incredibly helpful to you in battle and exploration; instead, sell the ones that are no longer beneficial to you. If you already have warm clothing for cold weather, for example, then you can sell Cold Resistance meals as they won't be as crucial to your survival anymore.