The cost to build a website like Amazon/Ebay/Etsy/Airbnb is hard to figure out easily as the cost of building a multi vendor ecommerce website of such big proportions involves multiple factors like design expenses, annual hosting expenses, platform based development, payment gateway integration and much more.
Though all the websites a huge portal and is not easy yo structure out, better you can go with the readymade solution because it is easy to customize, supports best features, time consuming is less.