Well if we're talking all in one single "illness", then just short of $4K for our last black lab. Still have no idea what he got into. Came back home after being gone a week, adult daughter was watching our dogs (4 rescue labs) at our place for the week. Noticed Coal (black lab) was acting "off" a day or two after we're home. He was a "counter surfer" and would snatch things off of anywhere and eat them. Only thing we can think of was he took or got into something that daughter brought with her. Daughter was devastated by the whole thing, so I never did fully "interrogate" her over it.
Part of the issue was we didn't know what we were fighting with him. Started with regular vet visit, then local animal hospital, ended up days later at the ER animal hospital in Fargo. Only reason the cost stopped at $4K was he died while we were still trying to figure out what was wrong with him. Kidneys, liver shutdown, and he just wouldn't respond to any treatments, nothing came back as smoking gun in any of the lab tests or x-rays. That was 3-4 years ago. Still have no idea what happened, but the other 3 dogs never "got it", so he must have ingested something. Still miss him, would spend it all again if had to, and we were prepared to spend whatever it took to save him, he just didn't last long enough.
Our other "problem child" is a little yellow lab mix, he's not topped the single bill total (so far, lol), but he's been to the vet and local animal hospital several times over the years. I think he's up to $6K total, but usually not more than a grand or so at a time, and only one multi-day stay in the hospital so far (knock on wood).