What is the cheapest way to get from the Las Vegas airport to Fremont and back?

The CX bus runs between the airport and Fremont regularly throughout the day and costs $6.  That will be your cheapest option, and it's direct too. 

The cheapest wat is to take  the CX (Centennial Express) city bus. https://rtcws.rtcsnv.com/routepdf/cx.pdf

There are buses that run up and down the strip. The strip is visible from the airport- if it weren't for the fences, you could walk there. But you'll have to take a taxi or Uber from the airport to the strip, and then take the bus. 

They have shuttles at the airport their cost is the cheapest I have found. Uber unknown cost.

 i would call the airport and ask them

If they are flying again, Southwest Airlines most likely.

NV - LAS to OAK. You would have to land in Oakland and then travel down the freeway somehow to Fremont.

Now for me, jetBlue is my favorite airline. (Your ticket will cost about $450) Their smaller planes are fast, (1 hour-30 mins)! Like little rockets: https://book.jetblue.com/B6/AirLowFareSearchForwar...