Pet insurance costs?

Anyone have pet insurance? If so i have a few questions

Increasing cost as the pets get older or a flat rate?
What type of plans are out there and what do they cover/not cover?
Diff. costs for a pure breed VS a mutt?
Can i cover a dog/cat for its replacement cost?

I think Allstate offers a good "New Dog Replacement" plan...if you total your puppy they'll just get you another.