if using es as a reference, this is as low as $1.35 round turn ($0.68/side)
http://www.vcapfutures.com/futures-trading/futures-commissions.asp
http://www.vcapfutures.com/futures-trading/futures-commissions.asp

Quote from Tums:
if using es as a reference, this is as low as $1.35 round turn ($0.68/side)
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The above url reference only seems to require run of the mill "trading platforms".Quote from def:
the quote they give has a "*" and says "based upon 5000 RT's per month".
FYI, after 1000 contracts - NOT round turns, IB's sliding scale would be $0.65 per contract and can slide down as low as .25 per side for large volume.
You should also consider other factors such as interest earned on cash.