I'm not really thinking about the costs Jobbo - that doesn't really come into it for me. It's about saving lives.
I think the result of ANY form of safe passage will result in a massive increase in the numbers crossing Europe, all destined for the UK. We can only "Process" (Sorry horrible word but you know what I mean) a finite number per day so there might well be a humanitarian crisis in France with thousands waiting for their turn.
At the risk of getting all Brexity about it, I think the French, and to a degree, the European Union have a lot to answer here. If we assume the majority of those migrants crossing the channel entered the EU through the Med then the Migrant crisis is a European problem and you don't solve it by not stopping them crossing the channel and thus calling it a British problem.
Also, Rich is correct. The only way safe passage would work if everyone transported could stay - and this is obviously not feasible