How much should I be charging for WordPress sites?

Hiya, got about 15 years Front End Development under my belt but due to personal circumstances I've never charged anything to anyone for a website.

Lets say I build a bespoke theme for WordPress from scratch, set up a e-commerce cart along with all the server side requirements and then sort the SEO with decent copy writing. How much should I be charging in USD, British Pound Stirling or Euros?