Don't worry, that's very easy.
This is what I would do:
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Drop Item
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Events
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Conditions
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(Unit-type of (Triggering unit)) Equal to [YOUR UNIT HERE]
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Actions
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Set RANDOM = (Random integer number between 1 and 10)
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If (All Conditions are True) then do (Then Actions) else do (Else Actions)
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If: Conditions
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RANDOM Lesser than or equal to 1
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Then: Actions
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Set LEAK = (Position of (Triggering unit))
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Item - Create [YOUR ITEM HERE] at LEAK
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Custom script: call RemoveLocation(udg_LEAK)
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Other: Actions
LEAK -> Point variable
RANDOM -> Integer variable
Put in [YOUR UNIT HERE] the unit type you want to drop the item, and put the item itself in [YOUR ITEM HERE]. If you choose a Footman, and a Gold Coin, this trigger should make every Footman that dies to have a 10% chance of droping a Gold Coin.
You can change the value in the condition [RANDOM is Lesser than or equal to X] to modify the chances of dropping.
If you want only certain units to drop the item, not all of them, you could add them to a unit group, and when a unit of that type dies, check if it's included in that unit group.
Also, if the item is going to be dropped by, for example, just one unit; you could use the [Specific Unit Event] instead of [Generic Unit Event].
Maybe there's an easier way to do it, but this should work fine.
Regards