EDU domains are special type of global top level domain. When internet was at its inception, there were only few top level extensions including COM, EDU, GOV and NET. However EDU domains are not only worth because they exist for so long, but for their hard acquire. In fact there is no way you can acquire EDU domain for yourself if you are not some form of educational body and have permission from the governmental body. The screening process of an institution requiring EDU domain is lengthy and it requires quite a few hoops.
All that in mind, most search engines and Google in particular value a back link from an EDU domain more than the same ranked domain in other global top level domain extensions. Since whole Google algorithm is a business kept secret we cannot be sure how much more worth is one link from EDU domain from say page rank 5 link from different COM domain. The fact is however that if you can get EDU back link or have to opt for a sub page rank 5 back link from different top level extension you should indeed opt for an EDU one. Other search engines follow the Google suite on the subject. However read on to find out why the “best deal – dominate Google” type of EDU back link sales are just a marketing ploy and that you cannot really expect to be linked by a top level EDU root site.
The key to EDU back link is high page rank. It is obvious that e.g. Stanford or Harvard have high page rank. What is not obvious is that when you are actually buying EDU back link from numerous sources that you buy yourself a sub-sub-subpage link which does not rank anymore and has a value as your average page rank zero COM domain. This is the usual deal you will get from numerous places selling EDU back links. Suspicious guy selling three level sub domain back links from his own school blog. This is not going to boost your site ranking nor it will impress search engines at all. And most of all it can take you down in the search engine rankings due to poor quality of said back link. I am yet to witness a great content school blog on the internet.
The conclusion is that EDU sites rank higher in Google and other search engines due to their authority. Seldom you will find EDU domain in sub page rank 5 category. But to get a back link which would be worth more than low page rank COM or NET site back link you need to have at least niche content pass to get to first level sub-domain. It is almost impossible to get a back links from the site root – if you are a generous donation giver (and generous I use loosely here, think millions of dollars) you can expect an university to have thank you on their home page. However if you are going to invest millions you can get thousand of quality back links from other places and rank better than having been linked from EDU domain in the first place.