Sounds harsh and brutal, but I agree. To pay anything over 3 figures for an elderly cat (15-16 or more,) is batshit.
One vet we went to when our old girl (17) was struggling to poo, and wee, and was losing weight rapidly, and whining a lot, suggested she may have cancer, and she needs a 3 month course of treatment. She needs to go away for it, and it would be around £9,000. Vet suggested this without even knowing what it was!
I wanted to laugh at how utterly fucking ludicrous it was. Spend £9,000 on a 17 year old cat who is clearly nearing the end of her life?! LOL yeah right! And how the fuck she thought we had £9,000 to fork out for vet fees just baffled me. It was just after the first covid lockdown and MANY people were struggling financially. Bloody vet must have lost some work with the lockdown and wanted to recoup some money! 
And imagine sending her out to some strange fucking lab to be experimented on?!
I said a flat NO, and if she has not improved within 3-4 weeks, she can be PTS. She was clearly suffering, and yet they wanted to use her as some kind of experimental trial to make £9,000 from. Jog on. 
She did not improve, she got worse, and we took her to a different vet a few weeks later, who suggested PTS immediately. Said she had liver failure. Any vet who expects the owner to spend multiple 1000s of £££ on an elderly pet is taking the piss, and DOES intend to rip them off.