in our customs many people (especially the elders) say "I'd be a sacrifice", or "I'd be a sacrifice for you". Are such terms shirk, considering sacrificing is only for Allah. As far as I'm aware it's not meant literally.

Ustadh AbdulAziz al-Haqqan

No, this is not what it meant. Just like the Sahaba said to the Messenger, "بِأَبِي وَأُمِّي أَنْتَ يَا رَسُولَ اللَّهِ." Therefore, this is something that is used, and I'm afraid this might be waswas on your behalf.