KouriNick is exactly right about this interaction. Eldrazi Scion can be sacrificed for mana even with
Rest In Peace on the board for all the reasons KouriNick already stated and die triggers don't activate because the card never reaches the graveyard.
I feel I need also add that while
Rest in Peace is on the battlefield you can still attempt to put cards into a graveyard even though they can't go there. That interaction is highlighted with Eldrazi Processors like
Wasteland Strangler. Even though cards can not be put into the graveyard you can still attempt to do it and the card will leave
exile and return immediately to
exile. That will fulfill the requirement of the Processor and will still keep their graveyard empty.
117.12. Some spells, activated abilities, and triggered abilities read, “[Do something]. If [a player] [does or doesn’t], [effect].” or “[A player] may [do something]. If [that player] [does or doesn’t], [effect].” The action [do something] is a cost, paid when the spell or ability resolves. The “If [a player] [does or doesn’t]” clause checks whether the player chose to pay an optional cost or started to pay a mandatory cost, regardless of what events actually occurred.