Taking the tunnel to the strip is always a scam. It may be faster to get from the airport to any of the strip hotels, north of say, Flamingo Rd (Caesars, Mirage, TI, etc), but it will almost never be cheaper. If there's a lot of traffic, the taxi driver can take Paradise Rd, Koval Ln, or Frank Sinatra Dr.

Tell the cab driver when they pull up where you want to go, and "no tunnel". Last time I was there, I forgot to tell the driver, and sure enough he took me through the tunnel to get to the Monte Carlo. His tip was adjusted accordingly.