If you want to make some significant margin on your trade, You can not trade anything lower than ten bitcoin! If you can get up to 20 or even more you could constantly make more than $500 daily
Though not everyone has 10 bitcoins to trade with. Right now that is $80,000 which is likely unavailable to almost everyone right now. I guess it is indeed good if you can invest that much, but generally you can invest even as little as 1mBTC.