Trim(Len(Nz(Me.[qProductCost_subform].Controls("Cost")))) = 0
How is Cost defined? does it have a default value of 0? If so, should remove default. Also,try changing to something like this...
'Check if value is null or = 0
Trim((Nz(Me.[qProductCost_subform].Controls("Cost"),0)) = 0
I...