One way is to buy 1 YYZ-DFW rt, and buy 1 DFW-LAS rt.

Fly on Thusrday, say, from YYZ-DFW (AA non-stop), then fly DFW-LAS using the first leg of the second ticket. Allow plenty of time for the connection, because you won't be able to check your bags through (if any).

On Monday, fly LAS-DFW, and spend some quality time checking out the cultural highlights of Dallas. (Never mind.) Thusrday, fly back to Toronto.

Hardly ulytra-cheap because fares from Canada to dallas are expensive, but at least it's less than US$700.