The SWA price that was referred to was the last minute price. You didn’t mention when you were going. SW’s flights are cheaper when purchased (usually 2 weeks in advance). They also have a feature called DING that notifies you of even deeper discounted specials. In two weeks you can find roundtrip flights for 2 for $317. Flight time is a little over an hour, but then, as stated above, you have to factor in airport time. Driving time is about 4 hours each way.
One way car rentals are usually more than regular roundtrip rentals. The way I book a car is to go to the SWA site, and compare the various companies under car rental. That gives me a ballpark and then I go to the companies are listed and see if I can do any better. You can plug in your dates and answer your own questions that way, since we don’t know the details. You do not have to fly SWA to rent a car or hotel room on their site. If I were you, I’d also compare what it would be to keep the car for the duration of your stay, but factor in if the hotel you are staying at charges for parking. When booking hotels,I also factor in whether or not breakfast is included.
Keep in mind that the gas prices may be more enroute to LV. I know they are on I-5 in the middle of nowhere. If you are going now, it is hot as the hinges, so you have to figure in AC as well.