Issue #548 was solved but there is a comment worth a new issue.

Miles's notes about balance are persistent: The current skill implementation
always increases drop rates in 2% per Skill level plus a fixed rate of 10%.

If a detailed game balance was considered, this would take in account besides
the skill level, the bot/item/chest class, the item type (for example, equips
drops less often than other items, etc.) and some other variables.

In fact, if we used a detailed drop balance system, each item would have it's
own drop rate, which would be affected by the skill (it could increase 10% of
the item drop rate per level or in math, ITEM_DROP_RATE * BONUS_MODIFIER where
BONUS_MODIFIER is 0.1*SKILL_LVL)

While using a fixed amount might not be the best idea as pointed by Miles, using
a “proper item balance” is not an easy task. 

The game will survive using fixed drop amounts but for a more flexible game
balance those changes must be done. And it can be done in many different ways.

I extend this ticket with two proposals.

First is to use r/1837 and determine ourselves, the hard-coded way, rarity
multipliers for the item grades.

Second is to add a "rarity" field at item spec, optional, which influences the odds.

It usually requires some more patching because in the end, we are deciding the
final drop, not the drop chances. And this can be a little problematic depending
on how we do this decision. (ie. this may require more code lines)