Depends on what it would cost to build a C-5 now. C-17 cost in 2013 was about £200M/$260M. Present estimate for the BFR booster is $230M.
(The Interplanetary transport system (ITS) adds about $150M for a tanker and $200M for a spaceship - but they’re the extra bits to get to Mars, not for an earth-earth hop.)