Gas

What do you mean by a return trip?  I would think that you would be returning to Santa Monica from Ottawa. Driving to Ottawa and then back to Santa Monica would be what I would call a round trip.

Unless you know your car's average gas mileage and can predict the price of gas at the time of your travel, you can't tell how much the bill would be.

If I assume that your car gets about 25mpg and the gas price would be about $2.50/gal, then I would say it would be about $300.00 in gas to go one way from Ottawa to Santa Monica  Double that for a round trip

That is a very odd and difficult question since you would need to get gas prices for every place between Ottawa and Santa Monica.  That is a round trip of almost 6,000 miles.