Some monasteries and retreats offer free food and board, although donations are always welcome of course! I have heard many westerners pay US$4-6 per day.
Northern Insight Meditation Center: Wat Rampoeng (Tapotaram), Tambol Suthep, Ampur Muang Chiang Mai. Phone: +66- (0) 53 278 620. Fax: +66-53-810 197. Email: firstname.lastname@example.org. Website: http://www.palikanon.com/vipassana/tapotaram/tapotaram.htm.