I don't know a good way to come up with a number but it is definitely based on how response the page is to posts(likes,shares,comments), how the likes were acquired(paid or non-paid), the niche (fitness would obviously be worth more than a page for memes), and the demographics of the likes(gender,geos,ages).
I can't really give you actual prices cause it varies so much per page but myself I'd buy a 1 million likes meme page that was acquired from viral posts and made up of 15-30 year olds from all countries for maybe $2k whereas I'd pay $5k for a page with 100k likes in the health/fitness niche if it was women aged 35+ in US/CA/UK/AU and acquired primarily from fb ads.