can I decline to pay for last rites on resolution?

Yes — you can decline, but only by declining the whole "you may" effect; you can't play a unit and then refuse to pay for it.

Last Rites' conquer/hold trigger says "you may play a unit from your trash. (You still pay its costs.)" Because "you may" is the first part of the effect, the choice whether to perform the ability is made when the trigger finalizes on the Chain (Rule 383.3.a). If you decline, the trigger is removed from the Chain and does nothing — no unit is played.

What you can't do is pick a unit and then skip its cost. Paying the full cost is mandatory as part of playing it (Rule 357); if you can't or won't pay, the play is undone (Rule 444.2.a) and the unit just stays in your trash. So the only available "decline" is at the "you may" level — the parenthetical isn't an optional cost.