Syag: “i have heard that you are supposed to pay in relation to how much you “cost” the host”.
That is common in the non-jewish world where the bride and groom generally pay for their own wedding. The weddings are much smaller (most of my coworkers had 150 guests or less).
This has never been common in our community. You give what you can afford.
“Not really sure how someone cheshbons that”.
If you want to go with the cost per person all you have to do is call the hall and ask what is the price per person.