I can pay 3 pips roundtrip with the better brokers, with FB it's double, so unless I'm paying 50% tax (which of course it will be no where near) I will be ahead, plus I can use my losers to offset my tax bill for the following year, and will not be restricted to living in the UK (which is becoming the dumping ground for the worlds deadbeats).
I don't follow you. Follow on from my example, and assume a conservative 20% tax. The 750 dollars would be on profit of 3750. Divide that by 500 trades gives 7.50 per trade. So in my example, if you earn more than 7.50 average per trade, you're better off with the spread bet.