That's a very interesting question and one that's a bit hard to answer, but let's do some math to think about what might be going on...

First off, let's assume a reasonably good AdSense clickthru rate of 2%, so that means that the 1000 hits/day (not visitors, big difference, but I'll assume it's visitors just for this math) would produce 20 clicks. If the site earns $10/day, that means that each click is worth $0.50, right?

If the site earns $30/day, then those 20 clicks are each worth $1.50.

Having spent years as part of the Google AdSense program, I can tell you that $1.50/click is a pretty sweet payoff for advertising and while I suppose it's possible that there are keywords in the Unix space with that kind of value, I'm a bit skeptical.

So let's dig around a bit in the AdWords system, to see what kind of value Unix keywords have. Okay, so according to AdWords Keyword Tool, there are indeed some Unix-relaed keywords and key phrases that might command that kind of AdSense value. Consider:

Of course, no-one really knows the exact percentage of the AdWords cost per click that is paid to the AdSense participant, but if we assume 20%, well, then indeed we might be able to get $0.50-$1.50 per click AdSense ads on a Unix-related site.

There's a second issue, however. If the traffic is from Google, then it's important for you to realize that any change in the domain name record, especially a change in ownership, will cause the site to rank lower in Google for at least some period of time. That's right, Google pays attention to domain name records too, among the many factors considered when calculating search engine results placement.

Remember also that if you do nothing to the site, it's a sure bet that its position - and traffic - will decay over time too.

So, yeah, in the end these figures are probably legitimate (of course, you can ask the seller to mail you a screenshot of an AdSense report or equivalent) but I'd still be a bit cautious about overall valuation.