Disclaimer: never owned a horse, going by common sense here. Get advice from the others too!
For the first one, I would put his worth, but be guided by what you paid for him. If you got him for free or something, check online to see what similar horses are selling for.
For the last one, you should put the maximum you are willing to spend on your horse's vetinary care - if he coliced and needed surgery while you were on holiday, as an extreme example (I don't know just what your agreement says), would you be willing to pay tens of thousands of dollars, or would you ask that your horse be put down instead?
If you believe everything you read, better not read.