Hi, A friend of mine worked really hard on his website this year and he managed to get in the top 50.000 of Alexa. Unfortunately I can't tell you his URL (hope that is no problem) but it is a PR4 website with over 1k visits/day. His niche: internet related How much do you think his website would be worth without seeing the site?
It's not that much hard to gain, I've a few sites under 50k alexa. By Alexa ranking a site can't be estimate.
the niche is alot more important than the alexa is. warez sites easily get under 50k alexa within few months though they're still appraised under $1k while if its a webhosting affiliate site it might be couple of hundred thousand bucks