Lots of possible solutions. Here are three to choose from...
NOTE: I've had to replace [code] with [YourCode] to get round issues with forum code tags
Replace MyValue with your own variable
Code:
Select Case Qty
Case Is <=3
MyValue = DLookUp("item_price","Price Table","[Price Table]![item_code]=" & [YourCode] & "And [Price Table]![item_size]=10")
Case Is <=10
MyValue = DLookUp("item_price","Price Table","[Price Table]![item_code]=" & [YourCode] & "And [Price Table]![item_size]=20")
Case Else
MyValue = DLookUp("item_price","Price Table","[Price Table]![item_code]=" & [YourCode] & "And [Price Table]![item_size]=30")
End Select
Or ...
As above but using If ... ElseIf ... Else...End If
Or
Code:
Dim intSize As Integer
intSize = IIf([QTY]<=3,10,IIf([QTY]<=10,20,30)) 'air code - check the bracketing
MyValue = DLookUp("item_price","Price Table","[Price Table]![item_code]=" & [YourCode] & "And [Price Table]![item_size]=" & intSize)
Personally, I'd use the last of these