Trash for Treasure ruling needed

[Gort]

Can the artifact returned with Trash for Treasure be the one I just sacrificed for it? For example if I just want to trigger a entered graveyard effect

[Arix]

No. Targets are chosen before costs are paid, so by the time you're choosing what artifact to return, you haven't sacrificed anything yet.

[Gort]

isn't the first part of casting a spell paying the cost? otherwise I could target stuff without having enough mana?

[Arix]

No, and no.

[Twanos]

The best way I know to describe this without getting knee deep into a rules quagmire...

When you go to cast the spell you select the targets (soft target) to make sure they are legal for the spell you are wanting to use. If they are not you can't cast the spell. If they are legal then you move on to paying the costs and move the spell to the stack. Once the spell is on the stack, the game checks the boardstate for effects that will trigger because of the spell and/or targets. This is where targeted effects will actually catch something happening (hard target).

So in your case you have to soft target the artifact in the grave to make sure it will be legal before the costs have been paid. Once the costs are paid and the spell goes to the stack you can't change targets to the new one you just moved to the grave.