Personally, whichever startup did this is a douchebag and I wouldn't want to work for them regardless. I have never heard of an interview done this way. I am

Personally, whichever startup did this is a douchebag and I wouldn't want to work for them regardless. I have never heard of an interview done this way. I am familiar with and okay with a coding test, ok with a proof of your abilities, not ok with implementing code for the company sans payment. My 2 cents, is they agreed to a price and code so they should pay. Frankly, I'd ask an attorney to send them a letter stating as much on your behalf. Might cost you a few bucks but seems reasonable. If they still refuse, I'd seriously consider a public shaming, although I know this isn't likely a good idea, it just seems so appropriate. lol