So, I've updated the control source to this:
=Nz(Nz([Rate],0)*Nz([Acres Sprayed],0)*Nz([Cost/Unit],0)/(Nz(Len([Combo50].[Text]),0)-Nz(Len(Nz(Replace([Combo50].[Text],",",""),0)),0)+1),0)
I know - overboard with the Nz(), but this still returns the #Type! error. Control format is set to...