Flight costs vary wildly, depending on the airline, your seating, fuel costs, season, how busy an airport is, when you make your reservation, how much luggage you have, etc, etc, etc. You can find a flight from L.A. to Las Vegas for around $75 if you reserve a couple weeks in advance and it’s during the week (not the weekend) and there are no holidays or special events that week.
On the other hand, it may cost $300 for the same seat on the same airline for a Friday flight on a busy weekend if you reserve the day before. The only way to know is going to an airline’s website and entering the day you want to fly.
Minus connecting flights and delays, here is approximately how long flights take from cities with the busiest airports:
International
There is no magical trick to saving money on a flight to Las Vegas unfortunately. These basic (and for the most part well-known) tactics are basically the same no matter where you are flying: