How does Dota2 Item Value work?

Generally, 1 item you pick = 2 randomly picked of the same rarity or 3 of the next rarity, but since there's no scrap or crafting system, it's much harder to calculate. Usually, rares are and higher the only thing worth anything and you don't usually get too many of those, so the best way is to just look up trading threads or ask for price checks or spreadsheets.

It's all based on if it's store only or can get dropped in game combined with how in demand the item is. (Sange and Yasha, an uncommon, was worth 1 rare for a while, for example.)