For futures:
1) For individuals, I haven't seen $3 round trip, but I have seen near $8.
2) Would have to account for bid/ask spread on each transaction, which I think is up to $12.50. This is a minimum price fluction (tick size), which should be treated as a penalty or expense when compared to trading stocks where you can specify down to the penny.
3) A small margin is required and should be accounted for as dead money. For instance, futures trading of a principal of $10,000.00, or more, will probably require a margin amount of about $1,250. So, % gains and losses should be calculated on $11,250.00, not on $10,000. This is an added penalty compared to trading stocks (or ETFs or SPDRs) long, since no margin is required when going long. However, it is a benefit when compared to shorting stocks because that requires much larger margin.
Even at 3$, total cost to trade is probably not significantly less than the $15.98 I assumed in my analysis. I don't know for sure, though, because I've never traded futures. Also, I didn't account for margin in my original analysis of going long/short stocks. The margin requirements are much larger for shorting stocks than for futures trading. That much larger amount of margin sitting around is a much greater penalty (dead money) which should be accounted for when comparing one method of investment against another. For instance, one might compare an investment method where one always goes long with another method where where one goes both long and short. This (margin business) is one of the reasons I never short stocks or funds directly. I usually short the market by going long funds which short the market. That way all my money is invested and I don't have to sit on margin.
Market Data 
Markets 
AI
