There could be insanely expensive things for purchase, talking tens of millions of gold pieces. That's almost unfathomably expensive and would really put things in perspective, for instance, you may thing that Ebony Armor of Spell Absorption that sells for 10000 gold is ridiculously expensive, then you remember, "Oh that's nothing."
This isn't something that would be required of your character. In fact, it should be incredibly difficult to achieve true wealth in the ranges of millions of gold. The journey could start off small, like buying a mine in the tens of thousands, which you could choose either money, or raw ore returns. This would be great for smithing. Or buy taverns or shops, which would help you generate the required money for purchasing things like large chunks of land, islands or palaces for millions of gold. Buying a large chunk of land and having a palace built there would be fun. This would take both a great deal of money, and time to go buy for the actual construction of the palace.
Generated income could be tracked from all the mines, farms, vineyards, shops, brothels, etc... that you own. Once you reach a certain threshold and buy something in the millions, or say your palace completes, new quests could unlock. Quests that deal with either gaining or losing large amounts of money, like gambling rings, people trying to blackmail you and succeeding if you have shady things in your past etc.This kind of play would greatly validate taking the merchant perks, because as stated before, making money quickly becomes obsolete when there is nothing to buy. Taking the merchant perks would seriously need to be considered if you wanted to go the true wealth route.
After achieving true wealth, you can reflect back on that time when you thought said Ebony Armor of Absorption was expensive and laugh. Then you can walk down the street with your Fine boots, Clothes and Hat, feeling that you truly deserve wearing them.

