You probably thought $20 a month and not day, right? Anyway, it depends on the length of time its has been getting this amount because a buyer would want to know this rate has been consistent for some time. The sources of traffic is also an important consideration.
To determine how much the site is worth based on monthly revenue, it really depends on the niche and where the traffic comes from because you need to factor in the chance that the site might fall outta the serps, competition, etc..
How many sites have I seen mentioned on here where the site is making $20 a day and the person who made it wants to sell it? Almost always it's because the site is MFA or a proxy or on some kind of sketchy ground where the revenue isn't guaranteed to be consistent six months from today.
Yep, that what I meant: if it really makes $20 a day without work... why sell it? Anyway - the guy sold it already, so I suppose he had his reasons. Maybe it was in sketchy ground, maybe not.
Yep, no problem with that. As a potential buyer, it's good to know motives behind the sale. (besides checking the site and buyer's past trade history when dealing with site sales)