Materials

Shiny Precious...
Every mission, except Survey beyond the Wall, will reward you with Coins, EXP, and Materials. There are a dozen uses for these materials, but no way to drop some. If you ever have too much Iron, which you should, there is no way to compress or trade for other, more valuable Materials.

The Individual Shinies
There are twelve different Materials you can gain. Each are gained on Missions. In the Story Missions, only a few basic Materials, like Silver, Obsidian, and Radium. To get higher leveled Materials, you must either beat the last Story Mission or play online. Only there can you gain the Crystals, like dyes, and Precious Gold. Not Gold coins, but the raw ore. These can be used for a lot of things, but the developers should consider the buying and selling of certain ores. All are for a price.

Uses of the Shinies
Every single thing you synthesize will require Radium. Upgrading your blades, VMG, or your character also require other materials. To raise the skill level, removing the character level limit, you must spend Precious Gold. Each character requires more or less gold than others due to rarity of the character. Buying new weapons and gear can use from basic Obsidian and Radium to costing you every material in the game. Still, having a high-density blade can help you slaughter the toughest Titans out there. They each have their perks, such as being super light, or having heavy blades to fall on the necks of titans without a swing.

Upgrading your VMG will allow you to move faster, farther, and longer. You won't waste as much money and help you save. You don't have to be on their backs to slash their neck, nor do you have to be on the ground when you strike. (you have to be latched on and attack before you move or it's worthless) The high price will be worth it in the end.