Neko got up to 8.75 units Lantus before she turned it around. That was after getting to 5 units Caninsulin and the vet thought that was "the dose". Neko got her first reduction literally days before we hit the road for SRT (radiation therapy). Cats need as much insulin as they need. That can vary a LOT. In the meantime, I'll quote a phrase used by a vet tech who used to post here "be the tank, a tank always gets where it's going". In other words, keep increasing as per the protocol and you will eventually get there. We just don't know at this point where you'll find the dose that moves her. It's not uncommon to see acros needing 20 units or more, but also less. It really does vary.
The nature of insulin resistance is that it can be a moving target. In early summer of 2015, Neko was a fairly regulated cat. Then the tumour came back in July.

I was chasing a moving target. I'd get to greens, be happy with the dose, then off we'd go up the dosing scale again.
It's unfortunate that you can't afford the tests. I get that - they do cost money. But when and if you can do them, I would consider getting a prescription for cabergoline. It can impact the amount of insulin needed. We've had, I think it's now 5? cats go OTJ on it, and the vast majority go on a lower dose. Thankfully Lantus now has biosimilars available in the US that make it cheaper. But at some point the amount of insulin needed can also get pricey, so it's a trade off of insulin vs. treatment. But you need a diagnosis first.