Again, so sorry for your loss. My friend had to do the same after her horse dropped dead in the field one day. We got her instructor and the vet to do a valuation. As others have said, they listed his attributes and quoted current prices for horses advertised with equal abilities.
I have a lamp base made out of shoes. It was made from my Uncle's horses shoes. My Uncle died of cancer 7 years ago, so it is a lovely reminder of him.