The value of getting a front page on somewhere like Digg is the links that your site gains. Increased links raise the authority and trust of the domain and increase the number of phrases that it ranks for. To put a value on it, my best ever link haul was 18,000 links, at a realistic price of $30-$40 per link (if you were going to buy these) this is a return of $540,000 - $720,000
i wonder is it tough enough to achieve that? i usually never get over 500-600 diggs, and came few times on front page. But that really helped getting my sites indexed and gaining quality traffic.
A huge proportion of Digg users are site owners and bloggers. A story goes popular and if it's link worthy then a lot of them will write about it and link back to the original source.