I got mine through Broadstone- I'm working off of memory, but I believe mortality and major medical up to $7500 had an annual premium around $450.
For the horse's value, I used his purchase price. I think I may have had to send in a copy of the bill of sale to substantiate that claim. They do say that you can later increase the horse's value based on training, show record, etc. but you'd have to justify it. So odds are, you wouldn't be able to buy a horse for $500 and then insure it for $10k- even if it was potentially worth that much, you'd have to prove it through a show record or similar.
For liability- double check with your home owner's or rental insurance policy (you'll likely have to call them up to be sure) to see if they'll cover liability on a horse that is kept off property. I was very happy to find that mine does