The drop came despite a competitive marketplace in which demand outstripped supply throughout the month.
Overall average values dipped 1.5% to £4146, but performance against Cap improved to just under 100%.
Fleet values rose by £74 to £4835 in September while part-exchange values edged up by £22 to £2376.
BCA said price pressure in the nearly-new sector, where volumes remain very low, caused values to fall by £80.
In contrast to BCA, Manheim said average van values increased by 3.2% month-on-month in September to £4126, suggesting a return to the traditional seasonal uplift.  
Manheim said average age of stock was down one month to 51 months while average mileage fell 1074 miles to 70,559 miles.
The firm said a 20% hike in year-on-year values illustrated growing market strength.