Try SFO-LAS. Might be the cheapest M fare out of the Bay Area. (Intrastate flights tend to be higher...even Modesto will run you upwards of $600.) You can ask the reservations agent to book it in M and they can easily provide you with a fare quote. You can even ask them to jump around and check various point-to-point fares. (Too bad the Web site isn't equipped to handle this...)

YBM are the full-fare economy classes. These are fully refundable fares with nearly no restrictions, I believe. If they were non-refundable, or had more restrictions, they would be H, Q, V, W, T, S, etc...

Good luck.