Dragon Majesty or Shinning Legends which elite trainer box should I buy and why does Dragon Majesty cost so much?? thanks
Whichever you like more. Alternatively, buy both!
I enjoy them both, so I would say both.
The Dragon Majesty has 10 packs per elite trainer box and every one contains at least a holo.
If you are tight on money you could buy those 3 pack pin boxes from each of the sets instead.
I got these pulls from the LATIAS set the other day:
External Media
Look at the individual components and check which of those you like more.
Which set do you like more? Shining Legends or Dragon Majesty?
Which promo do you like more? SM70 Shining Ho-Oh or SM125 Naganadel-GX?
Which sleeves-print do you prefer? Ho-Oh or Naganadel?
What do you prefer? 8 or 10 boosters? 
Ultimately, it’s down to your personal taste, preference and budget.
But I’d go for Shining Legends because I like the promo card more… 