I would suggest ceramic cap is always better than metal cap , because ceramic is tooth coloured cap where as metal cap is not a tooth coloured cap . Average costs depends on which lab the dentist gives and vary frm clinic to clinic. Pls take your mother for the treatment as soon as possible because as u have said she has a problem from 23 years and still suffering so that she can have a beautiful smile soon :)
Next Steps
Need to visit nearby dentist as soon as possible since she is suffering frm 23 years
Health Tips
Need to brush twice a day and maintain your oral hygiene with regular dental check up atleast every 6month once