The cost of a BTC is mainly decided based on different factors:
Supply and Demand
Its growing acceptance in online markets
Acceptance in offline markets, shops, cafeteria, etc
BTC ATMs
Legalisation in different countries
Support and Opposition from major investors, developers, etc.
These are some of the reasons which determines the BTC price.
Source: https://www.quora.com/How-is-the-cost-of-a-bitcoin-decided/answer/Shebin-John-4