You can open a TikTok Business account for free. It does not cost anything. However, making and putting up an ad will cost you. They start at about $10 per CPM (Cost Per 1000 Views), and you must spend at least $500 on an advertisement campaign.
Also, different TikTok ad formats cost different fees:
Brand Takeover ads cost $50,000 per day
In-feeds ads cost $15 per impression
Hashtag Challenge ads cost about $150,000 per week