Do you have Event Tickets and/or Event Tickets Plus activated? If so, then the “Cost” field will no longer show up by design. The “Cost” field is now only there as a fallback if you do not have a ticketing plugin activated.
Do you have any further questions on this matter? If you’re trying to re-enable that field, that is unfortunately not possible without custom code which we cannot help with 🙁
If you do not have any other questions I will close up this thread 🙂