Thinks of it logically. If you earn 200$ a month from a site, there are two selling scenarios:
a) you need money fast. Then you can lowball and sell it for 10$ or 200$ or 400$ or whatever
b) you are trying to make as much money as possible. then you have to think a bit more longterm. think about 12 month. how much money will you make when you keep it for 12 month? how much work is it to keep the income level for the site? how much risk is there that the profits drop suddenly? so when you are 100% sure the site will earn 200$/month for 12 month with almost no work, you would be stupid to sell it for less right? same goes for 24 or 36 month. what you need to do is convince the buyer that the site is a safe bet. Less risk = more longterm planning possible = more value for your site. If however you need to work 4 hours a day two keep that income level, the site gets a lot less attractive.
Etc.