How can I sum the field of all the entries of one form?

How can I sum the field of all the entries of one form and show it in a field of another form like a "formula field" for example.

I have two forms, one with people and another one with bills. The bills form have also one field with the price of the bil, and another field with the name of the person ( of the person form).

I wanna do a form that shows me the sum of all the prices of the bills that have one person, for example.

is the solution a formula like "count"?? Which formula? Who it works?

Thanks a lot.