According to the Keflavik Airport wikipedia page, the only Star Alliance flights there area an Adria Airways summer seasonal charter from Ljubljana and summer seasonal service on Lufthansa from Frankfurt that begins May 2 next year.

Here is the full list if you want to peruse

Note: there is new service on WOW air from Boston (starting March 27) and Baltimore (June 4). That's probably the cheapest way if you aren't using any miles.

Just checked pricing from DC area, and it was $530ish next August. Not too bad at all.