It depends again... namely on the domain's conversion rate / on the traffic's quality.
The more clicks a domain can generate the higher is its value. So a domain with only 5 unique visitors per day can already be worth a good amount of money if these visitors generate revenue on a daily basis.
Especially type-in traffic is valuable while link traffic sometimes is low quality traffic, because type-in traffic converts best in my experience.
So it is really difficult to say what amount of uniques a domain needs to be worth anything. Traffic is always worth something and the more traffic your domain receives the higher is its value.
For instance, a domain that receives 100 targeted uniques per day, has a CTR (click through ratio) let's say 5% and generates $1 a day is probably worth $x,xxx when selling it to an end user or investor.
On the other hand, a domain that receives 1,000 uniques a day might only be worth $xxx or less if the domain's traffic does not convert well (i.e. the CTR is <1% or the CPC is very low).
I hope this helps...