The price of these coins changes over time, and the unit of currency they are priced in has also changed many times. Today we value these in US dollars when trading coin in the US. But these are priced in currencies all around the world because they are collected all around the world. 
While the price often changes the value of these coins has been extremely consistent over the past 2,000 years. Surviving plagues and famines, wars and pestilence, revolts and natural disasters, these coins have maintained value and demand. 
So not only are these collected all over the world (unlike most coins), these have maintained value unlike other coins.