I got pet insurance years ago after having two dogs come up with high medical bills within a short period of time of each other. I almost had to make a decision I didn't want to because of the cost. I didn't, but had to borrow a huge amount of money.
I first got Veterinary Pet Insurance (VPI). There was a limit for each illness/accident that incurred....not sure if it's still that way. My dog had Degenerative Myelopathy---the insurance coverage on that maxed out--the rest I had to pay out of pocket. They were quick to settle claims, though.
I now have Pet Plan, which has no incident limit, covers hereditary illnesses and some other things that VPT doesn't cover. Fortunately I have not used it.
Premiums are MUCH less on mixed breeds opposed to purebreds.
As far as expense--I would rather have the insurance and not use it than to not have it and need it. I've read that if you are disciplined enough to save enough money for major vet. bills (which I'm not)--you're better off....well, of course, it's a gamble, just like human health care coverage. With veterinary medicine getting more sophisticated (and expensive) every year--I'll pay for the insurance.