there is a tutorial on a item drop list which works for all neutral hostile units...
you can of course edit the event to something else like a specific unit type for example or a unit matching condition. like in my map i think i made it so that sappers drop items.
anyway in that you can add the items to the item pool, and when you do that there is a number to the right of the item id which is the chance.
it is out of the total chance which is the total of all the numbers' chances in the item pool. so if all the numbers in the item pool, the chances add up to 100, then you will get a percentage for each item according to what that number is. pretty easy. the difficult part is editing the items and the event. as for the event i make an event in gui and turn the trigger to custom text and copy that event to the trigger. as for the items you need to turn on display raw data in the object editor to the get the item id strings.
http://www.hiveworkshop.com/forums/f280/itempools-guide-35180/
edit: i guess you could do it for preplaced units too. you have to go make a gui trigger probably and (generated global) select the unit that is preplaced to be the unit that dies and then convert the trigger to custom text like i did with the unit type and copy that event over to the item pool trigger.