Not trying to attack this post, but using it as a jumping off point...
Fangraphs are the only ones so far who have tried to translate prospect quality into monetary value, both so systems can be rated overall and so it's easier to figure out fair deals involving both prospects and big leaguers. Their current rankings (updated frequently) are here:
https://www.fangraphs.com/prospects/the-board/2019-in-season-prospect-list/farm-ranking?sort=-1,1&type=100&filter=&pos=&team=
You'll see that it would take about $75M in additional prospect value for BOS to move from 30th to 18th. That is the equivalent of three top 100 prospects, two hitters ($28M each) and one pitcher ($21M). Right now Fangraphs has 106 prospects rated 50 or above, so it corresponds very well to a top 100 list (and everyone between #38 and #106 is exactly a 50, only the top 37 are higher). Once you go below that (so the 107th best prospect in MLB on down), hitters who are 45+ are only $8M value, pitchers $6M.
TL/DR: It would take three top 100 prospects to move BOS from the 30th to the 18th ranked system right now, according to Fangraphs' system.