Attending a prom can be very costly. Today, couples frequently share the costs of the prom such as purchasing their own ticket, limousine rental, and dinner if it is not being served at the prom.
It is appropriate and greatly appreciated to offer to share these expenses when someone invites you to the prom since it can be a very expensive evening for one person to foot the total bill alone.
If one person is paying the bill, it is the person who initiated the invitation. Again, it would be very nice if the other person offered to pay for any expenses for events held before or after the prom.
The offer to share expenses should be made at the time of the invitation so there is no confusion about who is paying for what.