its difficult to put a value on an .edu link
for sure, they are the most valuable backlink a domain can have across all extentions. also, they are difficult to get - especially for free.
I have several websites that i have tried to get listed on .edu's in their appropriate sections. However, I have found it difficult to even get in contact with whomever maintains the sites, as link sections don't seem to be kept up all too well.
I too have seen .edu links being sold for hundreds or even thousands monthly
Given all those factors - includig rarity and difficulty in obtaining - they are extremely valuable; in terms of monetary value, I have no idea!