
I have had a varied experience with end of lives. It's taught me that none of us can predict what's likely to spell the end, or what's likely to happen.
I've had end of life cost that has varied from nothing (heart attack) up to around £2k (fibrosarcoma that required an MRI to diagnose, at it presented (and even biopsied) as severe gingivitis). Recently, my 11yr old had a 6-hour seizure that required an overnight admission on a saturday night, drugs and blood tests, which got to just shy of £400. So far no recurrence, so hopefully no further cost for that, but she is costing me in ongoing for her other conditions - CBD oil and B12. My other oldie has spondylosis and is starting to cost me more in painkillers as it progresses. Currently she's on 4 tramadol a day, but I think needs something stronger. She's 13 and still going strong, so her costs will be increasing for a while yet unless something sudden and new happens.
IME the ongoing stuff is not *too* bad depending on the dog (e.g. River only responds to the CBD oil, so did cost me a small fortune in various painkillers before that was tried), it's the sudden, unexpected things that can cost the money and don't always mean a quick final act - my lab developed IMHA at 10 and that involved 18 months of treatment and monitoring, only gentle as it was a mild case, before it relapsed and took her.
Saying that, the dog that had the heart attack could have cost an absolute fortune - she had DCM but was on a study into vetmedin in pre-symptomatic dogs, so didn't have anything else but most DCM dobermanns end up on a cocktail of stuff. So how much end of life care might really varies with the individual dog and the condition being treated. And of course, in some cases as mamabas says, it's better not to linger so that cost is small.