Nothing is ever actually cheaper in IT
In my experience, nothing that has been posed as being cheaper ever has actually been cheaper. If you aren't paying now, you're certain to be paying in the future. Even if you ignore the cost of migration, you still have costs in the form of things like SalesForce's outage last week where you end up losing a lot of revenue while SF finally got around to your data.
The first thing I tell clients is that if the new technology is going to solve a problem you have and can't solve for one reason or another, go for it. If you think it will be cheaper, then they are better off not wasting their money and for them to carry on as usual and their money is better spent making small changes to what they have already (Like providing better training, get better tools, upgrade hardware, etc).