can baron get discount from Hunters' Circle?

Yes — Hunters' Circle's [1] discount applies if you play Baron Nashor to Hunters' Circle, even though Baron ends up in the Baron Pit instead.

The location you play him to is chosen during Make Relevant Choices (Rule 355.2), and cost is locked in during Determine Total Cost (Rule 356) — before any entering happens. Baron's ability is a replacement effect that only rewrites the entering event, not the play location (Rule 369.3; FAQ #10250, FAQ #12610 confirm the play location stays as chosen). So he was "played here," the discount is applied, then he enters the Pit. You must control Hunters' Circle to play him there, and remember it's only the first unit or gear you play there each turn.

This is my interpretation based on Rule 355.2/356 and Baron's replacement effect. No official FAQ exists for this specific interaction, and I couldn't find a rule expressly stating whether "play here" checks the chosen play location or the final entry location.