Cost of onlays and there necessity

[Tamsq]

Dentist says I need onlays because work was already done on the teeth he is working on. Says too much damage... I need onlays rather than regular fillings. Onlays cost 1350 a piece... But he will do them 50% off. Each one is costing 650. After all is said and done i will be paying 5000.

Does this make sense? Do I really need onlays? Obviously, I know you can't see my teeth...

*their