There are formulas that people use when selling websites, such as 12x monthly earnings, that you commonly see on sites like Flippa, but the bottom line is that a site is only worth what a buyer is willing to pay for it.
Any manual evaluation should take into account the revenue it generates, along with the traffic source (organic vs paid), monthly maintenence fees, domain age and any special factors involved, such as updates needed for an SAAS site, or ecommerce platforms.
Simple evaluation tools are garbage and seriously flawed, so it should be done manually anyway. I've seen Godaddy tools evaluate sites at $XXXX that aren't worth more than the domain fee. I also read a listing for a game site that had $124,000 in monthly revenue, but the expenses were $95000 in paid advertising, so that was a huge factor for any buyer to consider, as it was the advertising network that held the value, NOT the actual site for sale. ANY game site could have done the same thing with the same budget.
No different than buying an offline business where the business itself doesn't hold value if anyone can copy the same idea and just market it the same way.
So, when you list your site for sale just present all the facts and make it as unique as possible to attract buyers and they will bid accordingly.