I turned an island into a vanity top for a customer several years ago. She probably could have bought the top, cabinet, and installation for what I had to charge. Turned out nice though.

It all depends. Maybe you should do a search on Houzz, specifying “in photos” and look at bathrooms until you zero in on a style of vanity you like. Measure your bathroom space where the vanity will go so that you know the minimum and maximum length for your vanity. Then shop. Look online. Go to bathroom suppliers like QualityBath.com, Build.com, or even Houzz (specify “in shopping” in your search from the drop-down). Look at prices for cabinets like you liked in your style search. You can specify many things in a search on these shopping sites, including length and depth, top or no top included, how many doors and drawers. Once you have a design in mind, make up a drawing with your specifications and male several copies of it. Call local cabinet makers and ask if they will give you estimates.

Considering you can get a vanity + top/integral sink combo for $80 retail, it really just depends on your budget, the size, and look you want. Custom anything is certainly more than prefab, that is undisputed fact.