@Haagel  
No. Simply because we lack all of the information does not preclude us from making guesses based on the evidence. To say otherwise would be to dismiss all of science since it is based on the implicit fact that we do not know everything.  
Especially since this is a show with contrived cirumstances so the audience is given knowledge based on what is shown
 
Rarity’s gem obviously paid for the most and the CMC would have had to get into contact with a vendor one can assume that he was more aware of the value of the gem since e was willing to part with industrial equipment despite being given a diminutive gem by children.  
And as a vendor he would want to keep his profit margin as high as possible so it makes sense that the red gem would be the highest.

@Sjogre
 
That’s a really good point there. We never learned the value of the others gems. Claiming that the red one was the most valuable one is therefore incorrect. We lack information to be sure.

@Keith Mowz  
Ohhhh. . . . I’m an idiot.

@HamburgerTime  
Pointing out that the other elements gave more than generosity, from spike’s point of view.

*by far the most trouble.

What did Fluttershy do that was unkind? She gave him by far the biggest gem, which was appropriate because Angel was by far most the trouble.

it seems to me… that if Rarity were trying to short Spike she would have given him one of the smaller gems she had rather than the larger centerpiece gem.
 
Being right all the time is tiring.

@Tim Timsen
 
God…

@Psychopomf  
Meh. Four round trip tickets to the Crystal Empire along with a private room seems like more buying power.
 
Not Spike got to have any of them appraised, so it isn’t like he got the actual value of the ones he used.

Color of gems = value
 
Red gems are usually the highest in alot of universes. I believe the taste to dragons is like the opposite. Least valueable gem is the most tastiest

Hey
 
That bought THIS
 

Don’t forget the origin of that.  

That gem bought an industrial strength pet drier and was clearly the centerpiece of that dress.
 
“Quality over quantity, darling.”

Bitch likes gems.

Smallest gem = biggest price
 
The cmc bought a giant industrial dryer with that little gem.
 
Try harder.