Can you sacrifice a land to pay a cost during your opponent's turn?

Sacrificing a permanent as a cost is not restricted to your own turn. Costs are paid whenever you activate an ability or cast a spell that requires them, and you can do this any time you have priority — including during your opponent's turn (CR 116.1, 602.2).

The key requirements are: (1) you must have priority, (2) the ability must be one you can activate at that time (e.g., an instant-speed or 'any time you could cast an instant' ability), and (3) the land must be an acceptable sacrifice target for that cost (CR 118.12). If the ability has sorcery speed restrictions (e.g., 'activate only as a sorcery'), you cannot use it on your opponent's turn.

Sacrificing a permanent to pay a cost is governed by CR 118.12, which states that to sacrifice a permanent, its controller moves it from the battlefield to their graveyard. This is part of paying the cost and happens immediately when the ability is activated — it cannot be responded to separately from the activation.

Concrete example: Your opponent attacks you. In their combat phase, you activate Zuran Orb ('{0}: Sacrifice a land. You gain 2 life') by sacrificing one of your lands. You had priority, the ability has no timing restriction, so this is perfectly legal even though it is your opponent's turn.

In summary: as long as you have priority and the ability is not restricted to sorcery speed, you may sacrifice a land to pay a cost at any point during your opponent's turn.