TL;DR Version: A common law trademark is FREE. Registering with a state is typically around $100. Federal registration for a single class will be anywhere between $225 and $1,500+, depending on what you want.
TL,DR version: When you are in business, you want to avoid infringing on another’s trademark and you should protect your interests in your own trademark. Going through the process of registering a trademark helps resolve these concerns.