cost of the domain is: cost of time searching for a reg by whatever means, is cost of reg/domain, cost of renewal where applicable, cost of searching for a buyer/selling to another domainer.
cost to your buyer is cost of finding your domain, cost of buying it including negotiation time, cost of renewals, further related domains can have to be acquired, cost of website development, promoting/marketing costs, compliance costs, seo, cost of website maintenance, myriad related business, monetization, and branding costs,
that's probably not a complete list of cost for EACH domain. if you see the domain as just reg cost you'll register virtually anything, with utterly unfounded yet eternally optimistic reasoning,and i'm speaking from experience.
if you apply the full projected cost of the domain over time in your reasoning before you reg, then you'll only register or buy CERTAIN domains.