(This is a quickie note.)
If someone came to you and said “I want to rent out your mail server. I’d like to send out some emails from your server, and I’ll give you $N to do it,” you’d be suspicious and probably say no–unless you wanted your mail server to end up on email blacklists. In the same way, if someone comes to you and says “I’ll give you $N to rent subdomains, subdirectories, or pages from you. Just link to my doorway pages from your content,” I would recommend to say no as well. It can affect the reputation of your domain if you host doorway pages for someone else and then that other person creates spam on the pages on your domain.