For LAS-DFW, Sun Country is $104.75 one way. Round trip is $209 on Sun Country, AA, HP and National (advance purchase with Saturday night stay).
Southwest from a non-Wright state to Love Field makes sense only if you are not staying over Saturday night but the "legal" cheap fares require that, or if you are flying last minute. LAS is cheap, cheap, cheap, so Southwest between LAS and DAL almost never works out.