Costs can't target.

A 2/1 haste Ravenous Chupacabra for 1B to life feels bonkers for 60 card formats.

I had some ideas (based on existing official rules) about how costs can target, although it will probably not do what you intended in this case.

Static abilities cannot target anything (only objects in the stack can have targets), so in this case the target is the target of the spell instead (which is the corresponding object in the stack).

If a spell or ability has one or more targets and all of them are invalid when it tries to resolve, then the spell doesn't work, and will go to the graveyard. (See rule 608.2b)

In this case, the target will no longer be valid once it is destroyed, so it will cause Halfdragon to go to the graveyard from the stack. I think you cannot pay the cost if the target creature has indestructible (since indestructible means a permanent can't be desrtoyed), but you can target a creature that regenerates (since that is a replacement effect instead, which does not preclude costs from being paid) and the target will remain in the battlefield and Halfdragon will resolve. (Halfdragon will also resolve if the target is changed before it resolves; the new target will not be destroyed, because the costs will have already been paid by that time.)

Another issue with this is that the wording would seem to mean it has a target even if you choose not to destroy the target. (Rule 702.33g causes a spell with kicker to not have targets that only apply when it is kicked, but that is not the case here. Since the rules do not expect costs to target, there is not a rule relating to the case where an optional additional cost has a target, so maybe it is not entirely clear.)