Cities where you can find a $99 one-way ticket include Pittsburgh (trips in January through April and August through October), New York (January-May, September-October), Washington DC (January-May), Boston (January-May, October), Chicago (January-May, September-December), and Cincinnati (September-October). On the west coast, you can get to Iceland from San Francisco or Los Angeles for $129 one-way.
As with any budget airline, be sure to get the full picture before clicking "buy." WOW charges for all luggage that isn't a personal item. (That's $39.99 for a carry-on and $49.99 for checked luggage.) Additionally, there's a minimum of a $6.99 charge to select your seat. However, you can have your seat assigned to you at the airport for free.
Nonetheless, if you pack like a damn hero and can survive having your seat assigned, you could be on your way to Iceland for as little as $229.98.