I seriously doubt it's tied to drop chances, speed glyphs are always the most valuable.

This is about an 8.5/10 on the tinfoil scale, basically for the reason Short listed.

Ya i mean we want to use them the most so their price may go higher (demand)

There is one other thing that can raise prices (supply)

Artificial scarcity is a thing (diamonds) 

I think its a fair observation, on paper those glyphs should cost the same, yet one costs more

the funny thing is the speed glyph is realistically several times more valuable than the other two, possibly combined

Yup, and plarium knows it well, so how would one prove that the drop rates are equal, when even a years worth of drops can be called a small sample size. Furthermore during that single year would you notice if one glyph drops 10% less than others without calling it bad luck xD

There's no data on the different glyphs having different drop rates. 

But I think Plarium knows which ones we have 10+ of in our inventory and the only one most of us have 0 of cause we're instantly using them haha