Your profit depends on the btc price in 6 or more months, rather than fee. MtGOX btc is like combination of 6 or 12 months CDs and future contract, which you cannot withdraw for deposit terms. Also, you need to be patient against default risk and frustration for their nontransparent support, which is like seller of CDS or buyer of junk bonds.  That's why you can expect higher return than spot forex.