Most containers aren't going to Trafford Park or a unbuilt port near Salford though, so road is going to be used for much of the journey anyway, so you'd have to make the cost difference huge to the point of wrecking the economy to get most of them on rails or ships.
That depends, the cost of a container 'lift' has been falling for years, and since unpaid externalities on HGV transport are considering to be somewhere between 40p-£1/km, it wouldn't take much to make transhipment via the MSC cheaper for locations in Manchester and further East.
Certainly considering the very low line-haul cost of barges.
Certainly not enough to "wreck the economy"
Rail infrastructure may be ridiculously expensive, but a tidal barrage on the Mersey was estimated at £3.5bn 6 years ago, and isn't going to be built any time soon or probably ever.
Such a scheme has the advantage that it also produces power, whereas any of the rail proposals floated to improve links moving east from Liverpool will only allow Peel Ports and (apparently) Drax Power to suck subsidy money out of taxpayer's pockets more efficiently.
The massive earthworks required in the middle of the river to extend the canal would probably also have eight or nine zeros on the price tag as well.
So you think building what amounts to a concrete breakwater in the Mersey a few kilometres long is going to cost many billions of pounds? Perhaps a few billion at a stretch but certainly not more than ten.
They build such structures all the time, and it would not involve very large scale 'earthworks' at all.
It would be built of concrete sections floated in place and filled with suitable ballast.
EDIT:
Container Transhipment in Turkey apparently cost something like $80/FEU, and whilst I would expect the UK to be higher, I don't see it being
that much higher given how heavily automated it is all becoming.
Especially if floating offloaders or mid stream operation were adopted, you could unload from far larger container ships than the 13000TEU ones acceptable at Liverpool2.
EDIT #2:
An LNG terminal in Mexico had a 650m breakwater in 28.5m of water constructed for it, cost of $170m (part of the $875m project).
So the cost of breakwaters of this nature are clearly much much less than a billion dollars per kilometre.
We don't need to extend the constant level part of the canal, all we have to do is provide shielding from the tidal currents and some of the wave effects.