Prosthetic Legs and Arms costs are different.

In the US

Legs cost about $5,000 - $50,000 

Arms are less at about $3,000 - $30,000

 http://www.disabled-world.com/assistivedevices/prostheses/prosthetics-costs.php

However the average household income in the US is $52,762

This being said, if a person wants to buy a the better prosthetic, the majority of the US population will have only $2,762 left for the rest of the year to survive on.

To top that off the average 'life-span' of an prosthetic is only 3 years. Imagine spending only $5,000 on one leg every 3 years for a life time. Say maybe 20 - 75, you would spend overall $90,000.