Buffalo to Las Vegas return for $200 all in on United
Sample dates include April 10 to 17.
You can check on the united.com website, or look at united.ca for prices in $Can. I'm unsure when the sale will end.
Edit: As found by golfer29, this fare is available on selected dates for all of this year and early next year. Use a tool such as Kayak or ItaSoftware to look for dates.