The "Value" field pulls in the log regression of the average % of salary cap value provided by those picks in excess of the first undrafted free agent. That is to say the last pick of the 7th round may provide 0.7% of the cap value in an average season of their career, but they only provide 0.003% excess value over an undrafted free agent. Only the 0.003%, in that case, matters for the trade because that is the extra value provided over what the other team can achieve for free with the next best option (e.g., a UFA).
That will have to be it for the explanation as I need to get back to writing about 6,000 more words for my project on this very topic.