Thurston County used to have a "typical land development costs" on the building department website. This was 5 or maybe longer years ago, and it was then about $60k or more.

Cost does not equal value. It is not unusual for an owner built residential property to exceed market value. You are doing one project.

Don't forget to watch for pocket gophers.

You might find it easier to buy a parcel with an old MH have that removed and build new.