The prices of TLDs are in part determined by the price a domain registrar, like IONOS, is charged by registries. Registries are organisations such as Afilias and Nominet, who control certain top level domains and set different prices for each one. When you purchase a domain name with IONOS, the registry responsible for that TLD charges us to reserve it. Therefore, the higher the price a registry sets for their TLD, the higher the domain registration fees will be for you.
The difference in price between TLDs can be for a number of reasons. For example, a registry's own operational costs may vary, and some domain extensions may incur additional support costs. In addition, the price of many new TLDs are unregulated and so for-profit registries can charge what they wish, according to supply and demand. However, many older domain extensions such as .com are regulated and have a fixed wholesale price. All of these factors can affect the final cost when you come to buy your domain name.