Joint Math Meetings (the American Mathematical Society (AMS) and the Mathematical Association of America (MAA) joint meetings) national conference to be held in Washington DC on 5 to 8 January 2009. Student member registration is $30. Annual AMS membership for college students is $40. There does not appear to be a student rate for MAA (which is the research arm of math societies, in any case).
See http://www.ams.org/amsmtgs/2110_intro.html for details on the conference and to click through to either math society.