The power of Fling is roughly divided like this:
10 BP - Berries, Choice Items, Focus Sash/Band, Insences and other miscellaneous items (see in source)
20 BP - Wings
30 BP - Evolution Stones, Mails, Pokeballs, X items, other miscellaneous items (see in source)
40 BP - Eviolite, Icy Rock and Lucky Punch
50 BP - Dubious Disk and Sharp Beak
60 BP - Orbs (other than Life Orb), Damp and Heat Rocks, Macho Brace, Rocky Helmet and Stick
70 BP - Power Items, Casettes, Poison Barb and Dragon Fang
80 BP - Dawn, Dusk, Oval and Shiny Stones, and other miscellaneous (evolution) items (see in source)
90 BP - Plates, Deepseatooth, Grip Claw and Thick Club
100 BP - Fossils, Hard Stones, Rare bones
130 BP - Iron Ball
If you ask me, I think that the the damage inflicted by Fling depends on how much the item itself when thrown can hurt the Pokemon. As simple as that.
Iron Ball hurts way more than the berries as it weighs a lot more (I mean c'mon, would you be more hurt if an Iron ball bangs you or will you be more hurt if someone slams berries into you), Deepseatooth hurts more then Deepseascale as the former has a sharper edge, which is bound to hurt more if thrown at a Pokemon.
Hope I helped!