Right now you have to catch almost 10 US$ price difference just to break even (profits go to commision) on one buy/sell cycle.
it's about $3,50 on fxopen for me
that's 0,5%
on forex it get's as low as 0.0025% (2.5 units of base currency for 100,000 on fxopen's ecn for example)
also spread on forex get's as low as 0,01% (one pip), while in BTC you often have 0,5% spread