The Legacy play makes sense. It’s a one-of in Legacy Sneak Attack sideboards, since it is technically possible to cast Emrakul, the Aeons Torn and take advantage of Kozilek’s Return’s second effect. But most of the time this is a Pyroclasm for one more mana that also happens to be colorless - meaning it dodges things like protection from red or protection from colored spells, which can be relevant at times. We’ve also seen a side-boarded copy of Kozilek’s Return in the occasional Miracle Control or OmniTell deck.
The Modern play is a bit more confusing. Primarily, Kozilek’s Return has been seen in Grixis Death’s Shadow Aggro decks. Again, it’s basically a colorless Pyroclasm for one more mana, as there is no Eldrazi to cast in that deck. But, yet again, being colorless helps against certain creatures, such as Burrenton Forge-Tender, which sees some sideboard play.
As a one-of in Modern and Legacy sideboards, and only barely any Standard play towards the end of its Standard life, it’s hard to see this card going much lower in price. This is definitely a good card in certain Eldrazi decks to help clear the way for the Titans such as Ulamog and Emrakul. Its exile ability is certainly decent, too. Of course, the main thing helping it keep its value is that it’s a mythic rare. Plus, being an Eldrazi-themed card, its price is propped up a bit, as well.
While it may list for $5, no one is paying much more than $4 for Kozilek’s Return now. It could hit as low as $2 or even less as Oath of the Gatewatch stock is liquidated around October 2017. But it may be worth holding onto if you already have copies. If it falls really low, it may be worth picking up a copy or two for the long run, as there is a chance that this card gets adopted by more Modern and Legacy decks in the future.