To whomever was saying stocks don't trade for a percentage, they definitely do trade for a percentage -- you just don't see it because you don't trade on open book exchanges.
Real stock markets do charge a fee, and pass some on to market makers who speculate on the spread. All this of course doesn't show up to you if you click buy 100 shares of amazon at 'market'.
US Equities has the smallest cost of trades worldwide, roughly 65 basis points. European exchanges for equities is 90+BP, and others go up from there.
I suggested 90 bp to the Gox master, but 65bp seems eminently reasonable.
(And, p.s. volume is down dramatically, yow! Should be an interesting few weeks -- I wonder if volume / price will trend back up.)