What Is Priceless?

The sale of the last privately owned version of Edvard Munch's "The Scream" has me thinking about the old Mastercard commercials where they list a number of purchases and the cost of each, only to end with something emotional that gets labeled "priceless."  What is meant by that word? 

We talk about cultural treasures, like great works of art, as priceless, yet they do sell for amounts.  Similarly, your child's happiness is important, but wouldn't you skip that ballgame or that ice cream cone if someone offered you a large enough sum of money or if you needed to in order to keep your job that pays the Mastercard bill? 

Theologians have long had a debate about the God's eternal nature.  Does it mean that God is present at each moment of time or does it mean that God exists outside of the realm of time?  If it is the latter, what does it mean and why use a temporal predicate to indicate it?

Similarly, if we say that something is priceless, does that mean its cost is infinite or it exists outside of the realm of cost?  If it is the former, then no amount of money would cause you to forgo it.  If it is the latter, what does it mean and why use a financial predicate to indicate it?