BRT doesn't have to be Guided- in some places in the world it consists of bus lanes up the middle of roads with "station" style bus stops, complete with over-bridges.
As you say, fundamentally BRT is just a s****y bus with the odd dedicated lane. It's still a bus though, and as can be seen form the cambs guided busway travel times, generally doesn't offer that much over a conventional bus apart from a huge £150 million bill.
There's no technical reason why rails should have cost any more than those huge concrete beams to lay - you could have had 'pact' track which is essentially maintenance free for less. Nevertheless on such a lightly loaded line, the track would have needed very little maintenance. The (comparable) Skegness line is currently having track relayed, most of which dates from the late 1940s/early 50s. The last substantial bit was laid in 1962 and the signalling system dates mostly from Victorian times!
You do need a signalling system (though not a very substantial one in this case), but then with a bus you need more drivers comparably.
Most of the costs with public transport are staff costs.
In the end it will come down to putting bums on seats. With no journey time improvements, and no frequency improvements - indeed such a corridor it is doubtful with you even need the current service, the effect would be very marginal. No way would St Ives to Cambridge support metro style frequencies for any mode, the demand simply isn't there. Rail is generally better at buses at modal shift and at least you would have had better journey times and the option for freight.
The scheme is obviously dreamt up by politicians and civil servants, with ideology in mind - as it so often is with these things.
So confident are they about this scheme, they are still planning on spending £1.2 billion on a new relief road which will link the A1 and A14.
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