Buy Section Prices

By discovering how many duplicates are needed to upgrade Quality,
it was possible to create these charts showing the prices of individual items.
The charts are divided according to rarity,
then element and according to item type.

The first column shows the price per item,
the second column shows the price for the items
needed for the maximum Quality level.

The first column for Total shows the price for purchasing one set.
The second column shows the price per set at max.

Common and Rare items are priced in coins.
Epic items are priced in gems.

Legendary items cannot be obtained in the Buy Section.

When we know the prices of individual items,
which vary, we can assume that each item has
a different chance of being obtained.

I believe that the lower the amount, the easier it is to obtain the item. And that could also apply to drops.
This theory led to the creation of the chart
Comparison of potential drop rarity of each purchasable item.

The color indicates the item's element, number is price.

For better orientation:
Electric
Fire
Poison
Cold

Since Legendary items cannot be obtained by purchasing, 
but can be sold, their potential rarity can only be determined from Sell.
Because the item must be Q1R0
(base item - Quality level 1, no-Reinforced),
 it is difficult to find players who have such an item and are willing to provide information.

However, from the data I was able to obtain, we can see that each item is sold at a different price, and therefore has a different value, so we can assume that the drop chance is also different for each legendary item.