Are you getting a good short balance interest rebate at these commission levels ($0.003/share)? I pay more ($0.009-$0.01/share), but get a short rebate, such that the net of the two is approximately 0 (i.e., the total short rebate basically offsets total commissions). I have a market neutral portfolio, so my short balance is always significant. Wouldn't work for a mainly long portfolio. Also, not prop trading. I have a prime broker.