PF,
Playing with your maths - 10% less pax is probably 10% less yield, but FR occupancy is usually in the 70s, not the 90s. There will also be a slight reduction in fuel costs - but that won't be 10%. On a 189 seat 738, that's 19 seats off the market - at £30 average, or £570 per sector.
What if CVT offered a £5 per head discount over BHX? On 170 pax, that would be £870 + a small saving in fuel. So using a quick back-of-envelope calculation, it may be possible - no doubt MOL has his beancounters who would have complex spreadsheets to work it out. Add to that the possibility that CVT has some very cheap car parks to fill, compared to BHX's gleaming white multi-storey structures. That's where the key differential is between the two - and that is more important to the punter than public transport.
But to answer your question - from the very heart of Coventry, the bus would be direct to CVT - about 20 minutes. Walk to COV station is about 10 minutes, train another 10, then that people mover swings it against BHX
But seriously - we're talking about a (very) hypothetical leisure passenger - I don't think anyone is expecting business routes - so they would travel from across the Midlands.