Ask HN: Working as part of an interview, for free?

I recently applied to a startup for a development position. I was told I would do a "trial interview" which was really just a feature implementation they needed. I was sent an NDA which also talked of when to invoice and had "rate: TBD" which i put in my rate next to. the spec was large, and I wasn't expected to do all of it, but I implemented the core of it, which was nontrivial; it took me over 25 hours to implement.

Now, I just talked with the boss today, and he said I wouldn't be paid for that time; that's part of the interview, he says. But I sank more than half my work week into this, and thought this was just a 1099 until i may or may not go on full time, because i was delivering a business feature and not just doing an arbitrary test.

Anyway, what should I do? I'm not so worried if I get the job, but if I don't then I feel like I've been cheated.