You can price based on eng time saved, but the rejoinder will be that the devs use test time to answer emails, respond in slack/jira, etc. And to some extent that's probably true. I do have some very strong advice on how not to charge: (1) do not charge based on your costs (customers won't care), and (2) do not charge based on fine-grained usage. The latter is important because if a VPE signs a contract for $3k/mo, h…
Counterpoint: I have personally seen companies of the size OP is talking about run into unexpected big jumps in spending on key infrastructure and tooling because they made some architectural change and then suddenly realised it was going to run the tests 10x as often or something. Without exception the response has been twofold. First they did take the immediate financial hit because they couldn't afford to be without a critical system. Second they diverted significant resources to getting the usage levels back under control very quickly so they didn't have to rework that critical infrastructure as well.
If anything, since bringing the costs of some important infrastructure down is exactly the problem OP's tool is solving and its value is directly demonstrable in terms of time and therefore money saved, your general argument should be a positive sign for OP that they probably have a viable business model.
I could buy an argument that tiered pricing might work better for OP than metered if someone showed up with data supporting the theory. But metered pricing seems to be working out pretty well for a lot of popular cloud-hosted dev tools and of course is almost universal among cloud infrastructure providers. People complain a lot about AWS having unpredictable pricing but I heard AWS has still had some success breaking into the cloud hosting market and probably it's making enough money that Amazon might keep it running. To be fair that's partly because although AWS lacks safeguards that would certainly be welcomed by many it does also have a reputation for being very decent about support in the occasional "Oh s@£t" scenarios.