To find out the marketplace app development cost needs to know more than just that it has to be like Amazon. It depends on so many factors. The app needs a good user-friendly interface, the ability to pay, easy navigation, and a lot more. I think that marketplace development is definitely not going to be cheap, because it's a gigantic job. You better ask several companies about the price. It will be approximate because there are many nuances.
I'm not planning to do a marketplace, but I was curious to know how much it all costs to make. It's very cool that someone has the bravery to open a marketplace along with a giant like Amazon.