perhaps one of the toughest comparisons would be with CFD (contract for difference) trading, available in Europe.http://www.fxpro.co.uk/http://www.aktionaersbank.de/ueberall-handeln/cfd-handel/
offers trading for ~10$ per trade. you only have to allocate 5-10% margin of the position you take.
this would be like if you want to bet on EUR/USD falling from 1.25 to lower for the amount of 12500 USD, you only have to deposit 625$ margin and pay 10$ for initiating the trade. If the rate is 1.20 tomorrow, then you made 500$ profit and pay 10$ for closing your position. In addition you pay some 0.1% bid/ask spread on the whole amount, e.g. 0.05% from midmarket price at entry and 0.05% at exit.
So you invested 641.25$ (=625$+10$+6.25$) to earn 483.75$ (=500$-10$-6.25$). In case USD/EUR goes to 1.30 instead, you will be asked to deposit additional margin or otherwise the trade will be closed automatically at a loss. So effectively, CFD allows a trader to mirror 12500$ FX position at a cost of some 32.5$, including bid/ask spread, and allocating only 625$.
The bid/ask spread would be wider, perhaps upto 1% for metals like gold. For FX, there could be multiple price changes within 1 second, 10 seconds are like eternity especially as many high frequency algorythmic traders take part. Imho, BitShares shouldn't target any professional traders at this stage, because professional trading applications evolved over decades with thousands of developers and traders working on / testing / improving them in fast moving markets...