Good Morning Victoria!
I'm sorry my tomorrow didn't come as soon as I'd hoped. I'm including a screenshot of the spreadsheet I made for myself to have low carbs and low phosphorous foods. The cost shown was all looked up online by Amazon, Chewy.com or PetSmart, a lot of these foods appear to be specialty, so not everywhere carried them, but I was able to get prices, so they were available in the States.
The first two rows (in red) are what I used as a guide because that's the Crystal's boys prescription food - that would not be okay to feed to a diabetic. It's just there so I could compare prescription food to the food options to keep levels similar, as long as I could also get them low carb. The lowest Phosphorous is bright blue and any that came in under the prescription are in the lighter blue, the others are listed because I was willing to go up on the phosphorous a little if it brought the cost down and meant everyone could be on the same food. All listed are under 230 for phosphorous. All are under 6 carbs. The pricing is also color coded for my daily cost. The lowest is in bright blue and all under $3.00 per day are in lighter colors. This is cost based on one cat.
I hope it helps you!
5 pounds is really tiny for an 11 year old. But then I have some really large cats in my house. My biggest can stand on his back legs and reach up and his paws reach over the top of my hip bone. He's a giant! My Fabby has always been a big gal. When she got diabetes (before diagnosis) she went from 18 lbs to 12 lbs in 4 months. Has buttons also lost weight with her diabetes, or has she always been 5 lbs?
The advice above from JeffJ and Kris & Teasel is spot on. They are a couple of the experts here, of which I've read a bunch of their advice to know they are good ones to listen to.
Though to get a kitty to eat more than she feels like is a task. Fabby has lost another pound after diagnosis, and I'm trying to get her to eat more so she stops losing. I chase her around, spoon feed her after she's stopped eating on her own, add water to make a soup, sprinkled salmon on top of the wet food to get her enticed to eat more... lol... I haven't got it figured out just yet, but if your kitty will eat more, that sounds like it will be good according to the experts above! And maybe once you find a low carb, low phosphorous food, she may love it so much she eats more anyway!!!