But it is not the only way, if you send them back you do not need hotels, detention centres etc etc.
Obviously the best solution is to stop illegal migrants from ever getting here in which case you then don't have to deal with the problem, as you're previous post said that should then also benefit France as 10's of 1000's of migrants wouldn't then enter France to get to the UK.
I wonder how many enter France with the intention of coming to the UK and then decide to stay in France

I have also previously posted this but have no idea if the logic has in fact been put to the French as a way to deal with the issue and gain better cooperation
But herein lies the problem in that you do indeed need cooperation with the French

How hard is it to stop the boats

I'd say it's probably impossible to stop 100% of them but i'd suggest with the right will and cooperation then you should be able to stop the vast majority of them but again it needs the French to do their part and not just bank the cash and then make weak gestures that hint of doing something.
People regularly drown trying to make the crossing and i'm sure that all political leaders would agree that this is unacceptable, the only sure way to avoid this is to stop people attempting to cross and i'd have thought that the French were legally liable under French / EU law if someone does drown

It's quoted along the lines that they can stop boats if they think there is a danger to life however i'd say this actually applies to every boat so surely putting legal pressure on them is the best way to sort it out

I previously suggested the swimming pigs idea (OK it was a joke) but surely these boats could all be punctured while they were only in a couple of feet of water waiting or just starting to load up with people, alternatively simply foul the motor / propeller with a piece of rope / net then tow it back to shore
All it needs is some out of the box thinking which is easy, oh and cooperation from the French which isn't