The least expensive offering I could find was $15.00 at ToysRUs for the standard game... and for those who are really really into it:
A Luxe Version for $44 at Entertainment Earth, which features a premium rotating gameboard, with special tile racks keep your smartphone at your fingertips. Take 7 tiles and create words vertically or horizontally that connect to previous words when it's your turn. If you use all your tiles first, you get to add the points from your friends' remaining tiles to your own score. Need to validate a word or keep track of your score with your phone? You're plugged in! Play the Words with Friends Luxe Game! 2 to 4 players. Ages 13 and up. (Smartphone sold separately).