40 minutes if there's no traffic sounds about right. We have flown into Ontario a couple of times now, like you we saved several hundred dollars doing so. Both times, we flew in, we landed around rush hour time and the traffic moved pretty well, it seemed more backed up going the other way. Don't know if that was just luck or the usual way the traffic patterns work. Then going back, we took early morning flights and it was the same way. We were lucky in that my father in law transported us but I want to say a cab was quoted to me as being around $85 each way plus tip from there to the
Disneyland area.
Ontario is a nice airport. I think it has a lot of carriers like UPS and Fed Ex, so it didn't feel crowded in the terminal. It was a breeze for us getting around and through security.
It is much easier to deal with than LAX and to save $100 per person over SNA, to me its well worth it. You will pay more for a cab or shuttle than from Orange County but not that much more.