Thus, it is difficult to know the price of developing a marketplace. After all, prices for such things are formed individually according to your wishes for functionality. But I understand your reluctance to consult with managers when you are not going to start the project. In that case, I can recommend you to read this https://www.codica.com/blog/how-much-does-it-cost-to-build-marketplace-website/. There you can roughly calculate the cost of development.