Rather than using Bountify to get completed projects it's probably best to ask for help with specific algorithms or approaches.
People answering could be those wanting to learn about something (eg, someone wanting to learn a bit about matplotlib) or an expert who wants to encourage best practice. Or someone wanting to build up a reputation for a resumé.
I agree that when compared to actual wages it's very cheap. The bounty is more a way of saying "thanks!", of buying someone a coffee for their help, rather than paying them for their work.
This usually works when the guy giving the bounty is not just another random guy on the internet.