That is a a low quality cheap prefab price, not a custom price. Custom is easily double that.

Our 10' custom walnut vanity from a local cabinet maker here in the Bay Area cost over $15,000 with tax and delivery. This cost doesn't include installation, counters, sinks or fixtures.

Not custom, and I believe the widest available is 72 inches, but Room and Board has beautiful well constructed vanities. You can add a custom countertop if you prefer. Easy to see online or in the SF showroom.

Look on Etsy! There are some good deals on there. Here's an 82" for $2500 in Walnut but you can email the seller since it is custom anyway https://www.etsy.com/listing/943144894/new-hand-built-mid-century-style?click_key=6d46aa8dff41f9d3899b8b72cf524d9baf470c75%3A943144894&click_sum=3a52f11c&ga_order=most_relevant&ga_search_type=all&ga_view_type=gallery&ga_search_query=90%26quot%3B+vanity&ref=sr_gallery-1-7&organic_search_click=1&frs=1