A plot of the ratio of prices MtGOX / Bitstamp since sep/2013:
http://www.ic.unicamp.br/~stolfi/temp/2013-09-01--2014-01-30-MGOX-BSTP-ratio.png
The prices are weighted mean prices in each 1-hour interval, obtained from http://bitcoincharts.com/
The labels on the X-axis are approximate months of 2013; "13" is jan/2014.