You definitely don't need frontier models for all this work.

And, in fact, I got it from £2500 to £150 by careful model selection (I ran a trial over a small dataset with different models until I found the least expensive/capable model that would reliably do what I needed), using a local model for one step, and paring down the inputs as well so the token count plummeted.

You don't.

Once you factor in cost of electricity and depreciation of hardware, it is more expensive to run locally.

There are many arguments for running local but cost isn't one of them.