There are some factors that could decide the prices of shinnies, among them are
1. The amount of same shiny in game
2. Design of the shinnies
3. Usefullness of the shinnies ( in pvp
and pve)
4. Personal preference of buyer ( like
some collector collect starter
shinnies, specific colour shinnies,
same type shinnies, etc)
5. How good its iv and right nature
6. How many people in game want and
search the shinnies
7. Rarity of the shinnies ( if theres pre
evo shinnies that have tier more
lower than its evo, the price usually
will follow like its pre evo since you
can evo it into same shinnies )