Well if you want some numbers - I was on the board of some real-estate business with around 350 units. They had this ancient website with a basic web form for potential tenants. Just a simple application where you entered your name and information, what type of apartment you'd like, and that would get sent to the operations manager as a PDF file. It was probably made in the early 00s.
The owners wanted something similar, but new looking - so really no added new functionality. Just a design do over, along with better hosting (the webpage was being hosted on the local server of the original designer, some retired dude in his 70s. If you wanted any changes, you'd have to call him and he'd apply changes to the HTML files).
Our cheapest option was a local student who'd charge us $3k for the job. But the owners wanted "24/7 support", so that went out the window.
Next in line, a local design shop quoted us around $7k. Still, no VIP support or whatever. And we'd still have to deal with hosting ourselves.
I had gotten a bunch of other quotes in the same ballpark, but the owner apparently knew a guy in one of the larger IT consulting firms, and they'd offer to do the job for a cool $20k (or around that ballpark), which came with 24/7 support and hosting for a around $2k / year. So they went for that.
Remember, this was just a simple landing page with a typical signup form, and a "modern design". Nothing more, nothing less. Total visitors a year? Probably under 1000.
I can only imagine the prices if they'd want some web app with user registration, login, tools, and what not. I can easily see some consulting firm charging hundreds of thousands for that kind of stuff.