Lies are bad for a relationship. So, we have a policy...she doesn't ask and I don't have to lie ! Works REALLY well !
I have seen the odd twinge of surprise form her that leads me to believe that she's not really up on what some of this stuff costs. When I let on that I'd spent $400 for a used light fixture (for which I don't have the tank yet), she raised an eyebrow -but she maintained the code and didn't say anything.