We primarely discussed the user acquisition costs in this thread and I pointed out a huge conversion breaker in the acquisition funnel of mobile which just let the costs skyrocket. If users stick or not after being acquired is as important but another discussion (rentention & churn).
But the funnel till this point you mention is MUCH cheaper on the web than on mobile (where people also install and deinstall right away or never use that app). And this 'being cheaper' is the whole discussion around.