Hi there, and welcome. You change back the same way you changed to Lantus - you keep the same dose, and just do extra monitoring for a few days.
That being said, Lantus actually works out to be cheaper than Prozinc if you buy it from the Canadian pharmacy (not sure if that's an option for you?). If you are in the US, we can give you the link to the pharmacy many folks on Lantus use. they are safe and reliable and the shipping is pretty quick. It also looks like you're using an AlphaTrak which is quite expensive. You can save a ton of money by switching to a human meter. The AT2 test strips are like a dollar for each test. On a human meter you can get test strips for under 25 cents each which allows you to test more consistently without spending as much money each time. most of us around here use human meters.
Have you read the protocol for Lantus? I'm noticing that you aren't changing the doses the way that's recommended, and that is likely contributing to the results you're seeing. If a cat isn't getting into good numbers, you can increase the dose on Lantus every few days (they can guide you over there on exactly how to do that) and those dose changes get you to better results. Right now it looks like Sammie is likely a high-dose kitty and needs more insulin to get to some better numbers/better health. If you decide to come back to Prozinc, the guidelines for dose changes are similar, but even a little faster than they are on Lantus. you don't hold a dose for days and days like you've been doing. It's important to be testing enough to make effective changes in dose so Sammie can get into better numbers.
You are very welcome to come back to Prozinc and we will do everything we can to help guide you, but I would encourage you to consider sticking with the Lantus and following the protocol for that faithfully for a little while, as Lantus or Levemir are usually the recommended insulin choices for higher dose cats. Of course it's up to you, and if you decide to switch we are here for you.