Found this interesting tidbit. First Open Source airplane could cost just $15,000 (http://www.wired.com/autopia/2013/07/open-source-airplane-design/).
I think the conditional wording is the critical thing. It could cost 15,000, but I don't think it will. I'm not sure what engine they think costs less than $15,000, but I'm not sure I'd fly in it.
Light Sport Aircraft do not use regular aircraft engines. They use a modified automotive engine, Rotax engine are popular and cost under $3K.