The fees are published. You are probably looking at $160 for MCS and $399 for F. See here: https://www.virginamerica.com/cms/vx-fees#/upgrades

As for likelihood, I can't say I have the same experience as VegasGambler. I fly mainly BOS->SFO, BOS->LAX and their reverse routes and, as VX Gold I'd say I feel like I have a 75% chance of an upgrade to F...But the seats always seem to be gone at T-6 unless it's a weekend or red eye.