13,300 was yesterday's DJIA close, rounded to the nearest 100.
If you could buy $13,300 worth of the DJIA stocks, equal numbers of each, and sell 1/5 of a Sept YM, then you'd be out three month's interest on your 13.3k. but you'd gain the time premium on the YM, and any dividends paid on the stocks.
Note that the imputed interest rate is lower than most brokers' margin rates, so the buy and hold investor is better off buying a YM than buying DIA on margin. SPX futures might be a better choice, as the DJIA stocks yield about 2% and the 2% isn't included in the DJIA, but dividends are included in the SPX and OEX IIRC.