“You should have seen the look on his face. It was priceless!”

You played a trick on your brother and really surprised him. He made a funny face. Now you're telling a friend the story of what happened. You describe his reaction:

You should have seen the look on his face. It was priceless!

You should have seen (something).

People use this expression when they're describing something interesting that happened. When 

You should have seen his face. He was so confused.

You should have seen the dress she wore last year. It was completely over the top.

the look on (someone's) face

"The look on" someone's face means their expression:

The look on her face was absolutely heartbreaking.

I couldn't stand to see the look of disappointment on his face. I turned away with tears of shame welling up in my eyes.

(someone's reaction) was priceless

A "priceless" thing is something that is extremely valuable. For example, "priceless" jewels are so valuable that you almost can't buy them for any amount of money. 

When it comes to people, we talk about a "priceless" reaction when someone reacts in an extremely funny way to something. You can imagine that people use the word "priceless" to describe a funny reaction because they get such high entertainment value from seeing it. It's the kind of entertainment that you can't buy, no matter how much money you try to spend.

Here are some examples of "priceless" reactions:

• You see a friend who's reading a book on a bench. When you walk up behind him and put your hand on his shoulder, he gets extremely frightened and drops his book on the ground. You both laughed about it afterwards.
• Your high school English teacher was really strict and mean. One day you noticed a mistake that he made on a test and pointed it out in front of the class. He got really angry, and his face turned red. You thought it was funny and joked about it later with your friends.