We offered "usage+minimal base fee" (cost-plus) based first, then used that data to determine fixed rate plans based on the initial customer usage. We still offer both, but fixed rate outsells by far because of the reliable bill.. though it is generally more expensive for the consumer (as we take on the risk).

We find our customers (mostly education sector) flat out could not get variable pricing approved. There's probably many other teams and sectors in that boat.

That said, flat rate pricing is sooooo much easier to deal with... Especially if you want to pass through the cloud costs, and not use your own "usage determination"