As for Burj Khalifa, these are prices for tickets at the observation deck:
Adult (12 years +)
General Admission (Dated & Timed) -125 AED
Immediate Entry AdmissionAED -400 AED
Child (4 - 12 years)
General Admission (Dated & Timed) -95 AED
Immediate Entry AdmissionAED -400 AED
Infant (Below 4 years)
General Admission (Dated & Timed) -free
Immediate Entry Admission- free
You can book tickets in advance online at this site
https://tickets.atthetop.ae/eticketing/homepage.aspx It will save u a good deal of money