Quote from JackR:
It depends on the firm. At IB they automatically pay interest on your cash balance. They now pay the "Fed Funds Effective Rate" less .5%. Until yesterday they were paying LIBOR less .5%
So using the LIBOR rate for your initial capital they'd pay 5.15% on $140K. IB does not pay anything on the first $10K of your $150K. Thus, the effective rate would be about 4.8%.
Few retail firms pay this much. Some sweep your credit balance into money market funds. Very few pay any kind of decent interest. Trade Station, for example, pays 1.25% on amounts over $10K and I'm not certain they have a "universal account" as they use an outside clearing firm for futures (RJ O'Brien).
Jack