The cloud provider can raise or lower prices as they see fit. What prevents these for-profit companies to keep lowering prices and not raising them?
That said, there can be quite a bit of customer lock-in associated with the "premium" services, such as DBaaS, or DNSaaS which make it really difficult to switch off of a platform. That could be the hook at some point in the future to keep people paying the "AWS tax".
Of course, that only works if you use provider-agnostic services (e.g. raw VMs, services with standard APIs) instead of locking yourself in to some proprietary service/API, otherwise the switching costs might be prohibitive.
Short of that, the trend seems to be costs going DOWN. Amazon recently noted their 42nd price decrease.
The cost per performance ratio is always going down too, which translates into lowered costs for them and ultimately the customer. They also can't collude together (at least not legally) to raise prices.
This is different from non-cloud based providers how?
Renting an apartment places a rough upper bound on your monthly expenses, since in most places, the landlord is required to do maintenance and cannot raise your rent until the lease is up for renewal.
Buying a house a rough lower bound, since your mortgage can increase unexpectedly, and you are responsible for all repairs and maintenance yourself.
So, you can never know all costs with certainty ahead of time, but if you care about predictability, it's probably better to rent (ie, go with the cloud) than to buy, because it's easier to bound your costs over a given duration.