I had a similar thing happen to me when I was interviewing at companies right before graduating. One of the companies sounded like an awesome opportunity. They

I had a similar thing happen to me when I was interviewing at companies right before graduating. One of the companies sounded like an awesome opportunity. They were willing to fly me out to San Francisco to interview me. However, the interview consisted of me doing 3 days of pair programming on their application. After the initial shock of excitement I realized the cost to fly me out there was much cheaper than they could hire a dev for. I actually thought it was a pretty ingenious way for for a startup to get cheap work while raising money. As some people have already suggested I think there are some good ways to avoid having to do this while still getting an idea for the candidate. You can offer to pay for their time, you can have them reimplement a feature you already have, or as the article suggests work on an open source project that one of the parties are familiar with.