All depends how this site is making $1000 per day. With natural traffic the domain it self will be above 100000k. With alternate methods to get traffic, multiply monthly revenue with 4 or 6 months to come up with price. Also depends upon niche for which site is making above stated amount. If the niche is high paying and site has not attained full potentinal than you can get more money. Lot of combination. Anyway congrats for bringing your site to $1000 per day. thx