Who Decides how much domain names cost?

There are several parties involved in the pricing of domains, namely;. 

ICANN (and other policy or regulatory bodies around the world)

Registries

Registrars

Registrants

ICANN (The Internet Corporation for Assigned Names and Numbers) is a private corporation recognized by the US Department of Commerce and, more recently, other country regulatory & policy bodies around the world, to manage and coordinate the successful operation of (TLDs) Top Level Domain Names.

Registries such as Verisign, Afilias, CentralNic and ZARC operate TLDs at the very base level, setting wholesale prices and operating the root databases for each TLD. They generally set their prices only after notifying ICANN and receiving approval. Changes must generally be approved by ICANN as well.

Although there are exceptions, Registries are considered the wholesalers of the domain name business and mostly deal with Registrars. Registrars deal with the public at large and could be considered the retailers of the domain name business dealing with individuals, businesses, and other end users around the world.

So Registries, with the approval of ICANN set wholesale prices (at any amount depending on their business models). ICANN doesn't try to tell a registry how much they should charge. They do, however, try to encourage competition.

Registrars,  such as DNS Business are organisations that manage the registration of domain names for one or more registries. They then are allowed to mark up TLDs by whatever margin they think the market will bear. There are over 1000 ICANN Accredited Registrars around the world offering retail domain name registration to end users. It's a very competitive business and tough for a registrar to price their domains with high margins. After all, the competition is only a click away.

Finally, there are Registrants or Resellers, who are the end users who use these domain names for their business or personal use. A good segment of registrants are "domainers" who buy, sell, and trade domain names in the aftermarket. You can easily become a reseller by signing up and domaining on the TopDog App and start your own domaining business from the palm of your hand!

This is a robust part of the business that includes buyers, sellers, and brokers. These registrants can charge whatever they think the market will bear.