Ask HN: How to price domain?

Hello all my startup is looking to purchase 2-3 domains however they are parked and are up for sale. We've contacted the owners however, they are asking us to make an offer. Since we have zero experience in selling or purchasing domains to private entities so we have no idea where start set the initial bid at. We've gotta search stats on the domains and they are +1mm search volume per month. Is there some formula, system or site to valuate a domain value?

Thanks