What do I do with all this money?

I am still pretty new to this awesome game, but I've got a ton of money from mining minerals and selling all sorts of stuff.

I've bought the miner's helmet, and some dynamite and a few other things but it seems like there should be more expensive items. Maybe I'm missing something? I have 4 NPCs (Nurse, guide, merchant, demolition), do future NPCs have better wares? Does the merchant ever get new items? What's the point of all this sweet sweet cash?

Coins don't play a very crucial role in the game. However, there is more to buy than you've seen thus far.

Just a sample:

• The arms dealer sells the Minishark for 35 gold and also the illegal gun parts for 20 gold. The minishark is one of the most powerful guns pre-hardmode.
• Some items like Purification Powder can only be bought from NPC's. If you care to clear the world of corruption you'll need to spend a good bit of cash on this.

Also, NPCs will have new items for sale after you defeat the Wall of Flesh.

The thing I find the most useful is the demolitions merchant. When you start getting around to finding and breaking Shadow Orbs, 3-stacks of dynamite are going to be particularly useful to get to the Orbs, and bombs are and any leftovers are helpful to deal with the resulting meteorite. Also, when you get to the underworld, this can be somewhat useful for clearing large amounts of Hellstone to get to the bottom of the screen (though you'll obviously need to fill in as you go so you aren't left with a big lava channel).

Don't forget the merchant also sells Lesser Healing Potions - as you progress you will spend less and less time above ground to collection Mushrooms, and you'll want to craft full Healing Potions out of a Lesser Healing Potion and a Glowing Mushroom.

The Tinkerer can reforge weapons by giving them random bonus abilities. Random is key here: there are many possibilities for abilities and reforging can get quite expensive, so you will easily sink at least a platinum piece upgrading your equipment to get the desired abilities.

In terms of multiplayer some players accept money for items, and of course their rates can be very high. This seems pointless but the difficulty curve after defeating the Wall of Flesh is quite steep, and most of the bosses are unbeatable on your own without newer gear or a buddy to play with. So hold on to your money, it might land you a really great item that someone else isn't interested in.

And if you still got money leftover after doing all that stuff, you can always use the Coin Gun, which uses money as ammo, doing quite a lot of damage on platinum coins.

Edit: Wait a second, this question is years old, why did it appear on top of my feed anyway. Oh well.

tQuarella posted an amazing answer but, there is also a Platinum coin tier, which is 100 gold coins, Illegal Gun parts used to be 1 Platinum but is now 75 gold which is the most expensive item to buy. It is used for upgrading the Minishark to a MegaShark which is considered to be the strongest gun if you count the bullets per second. It is also the hardest gun to make considering the crafting recipe which is 20 Soul of Might(you get20-30 after you kill the Destroyer), 5 shark fins(get from sharks at the ocean), illegal gun parts(75 gold, bought from Gun Dealer during a blood Moon) and a Minishark(bought from a Gun Dealer 35 Gold). It is a good idea to save money and convert it to as much Platinum you can to be able to buy potions, bombs, grenades, and rare items from The Goblin and Wizard.