> How do you technically prevent a cloud vendor from changing their pricing?

Through regulating markets to ensure fierce competition - including things like portability, standard APIs, banning egress fees and similar lock in techniques, breaking up infrastructure (DCs and networking) from service providers. In cloud we have 3 vertically integrated mega oligopolies. That’s not a healthy market.

> data portability […] probably requires a technical solution

Yes, formats and APIs are needed for technical reasons, but it already exists (or fairly trivial to implement) and is not provided – sometimes actively obstructed – for business reasons. Imo interop is predominantly bottlenecked by social/business concerns.