if i sacrifice meticulous restorationist that is in my board to play a discounted dune devourer, can i play the meticuluos restorationist from trash?

Yes — Dune Devourer has a printed Energy cost of 7, so playing it satisfies Meticulous Restorationist's condition even though you paid 2 less.

Meticulous Restorationist reads "When you play a card with Energy cost [7] or more, you may play me from your trash for [C]." Effects that check a card's cost for any purpose use its printed cost, even if that cost was increased, decreased, or ignored as it was played (Rule 206 — the same reason Lux triggers off a discounted Sky Splitter). The discount from killing a friendly unit is applied while determining the total cost (Rule 356.4), so you'll pay 5 Energy + 1 Order, but Restorationist's trigger still sees the printed 7.

Timing also works: the kill is paid in the cost step (Rule 357.2), so Restorationist is already in your trash when the play completes and the trigger is checked — meaning it can find itself there and you can pay [C] to put it back onto the board.