Quote from Lucias:
These are the rates I found.. I assume you mean the mini and not full contract. You must add exchange fees. There can also be volume discounts. The overnight margins are set by exchange and are same across board. OEC allows $500 intraday margin. Day trading margins typically vary from $500 to around $1500. Most firms require 5k to open an account.
OEC 1.29 (free)
Tradestation 1.20 (+ platform)
MBTrading .95 cents
Infinity --- Not sure
InteractiveBroker .85 cents
I'd say IB has the best rates but I think they require 10k to open.