This is completely valid question. Web Development projects are normally estimated on basis of past experience, client's expectations and future potential of the project.
When you want to build a Wordpress project, most prolific way to evaluate is to identify unique pages and on basis of that, you calculate efforts. And later multiply it with your per hour rates.
In a case when you're migrating existing site from one platform to another platform, try to understand client's expectations in terms of SEO, Content Migration and on-page optimisation.
In estimation, you should also consider efforts to be spent on - Design, Front end development, Wordpress theme development, content migration, QA, Project Management, client communication and 10% of buffer time to help you if you come across minor change requests.
Also, it would be ideal to set the right expectations upfront with a client about number iterations you would provide in design.
Hope this helps.