Big ambitious singleplayer games are likely actually far more expensive to produce than most MP games, specially "arena" based ones with a limited number of maps.Don't make it $60 then.
Internal alpha/ betas for mp are the big ticket items, but nowadays players are more than happy to do it for companies, even chomping at the bits to pay to access buggy early test phases.
Story writing, script, voice over (in multiple languages), motion capture, HQ asset creation forn stages that are non repetitve, etc.
It's compounded by the fact that you can't have a community (open) beta test your SP game, because then your storyline is out.
I just really hope that MP/ GaaS types won't eventually kill SP games (Witcher 3 and others have shown there's still plentybof money to be made), because as seen with ActiBlizzard and even moreso Valve, the appeal certainly is there.