With 60K (USD) in the account you'ld be earning (LIBOR - 0.5%) on 50K, if
it's all in cash, and you just let it sit at IB. There's no interest on the first 10K.
One possibility, if you're going to be inactive for a reasonably long time,
might be to do an ACH transfer of your excess funds to a bank account, and
then use Treasury Direct to buy T-bills.
I think the first withdrawal each month is free, and subsequent withdrawals
cost 1.00 each. Haven't tried actually doing this, though, so check it out first.
Whether this would be worth it would clearly depend on what your bank charges for an
EFT too, as well as the time period and amounts in question.