SSRS does not handle the 'divide by zero' issue as gracefully as other environments. Assuming
a / b
you can say "iif(b=0,0,a/b)" and that'll do it. But SSRS returns an error for this, anyway.
Solution below.
=iif(
Sum
(Fields!QtyCost.Value) = 0,
0
,
(Fields!ActualCost.Value) ) /
iif(
(Fields!QtyCost.Value) = 0,1,
(Fields!QtyCost.Value))