Take into consideration of his age, experience, potential, breeding, etc. I would definitely ask for some advice off a trainer or someone who knows the horse and has experience. Just make sure that you're not asking someone who may want to buy him as they might say a lower figure. I would also look at adds of similar horses for sale to gauge a price range.
I have always ask an instructor for some guideline, And looked to see what else is on the market similar to the horse.
Another thing is to not get too fixed to a price as different offers from different homes may come in.
Hope you find a good home for him if you decide to sell x
So you take how much you bought for him. So whatever you bought for him is your minimum. And then see if you taught hims anything that might improve him and make him worth more money. Like laterals or half pass or pirouettes. Or he can jump higher then he could before. These reasons would make him worth more
I would talk to a trainer, but if one is not available, you go online and look for horses similar to yours in size, age, disipline, difficulty, experience, and level acheived and then price your horse similarily. It also depends on the market of your area.