Truthfully, its not super complicated. You just look at the last 40 years of draft picks, and take the average WAR accumulation of each pick slot and you get an average value of the pick. Yeah, it's not something 1 person could do easily in a short period of time, but there's lots of publications and websites that have already done that legwork, so it's pretty quick to look up.
If pick 12 returns a career average of 5 WAR, then getting players that give you roughly that in return for the guy you took there is fine. It doesnt truly matter if they are a reliever, multiple relivers, or some guy that has 1 or 2 average seasons then fades away into nothingness quickly.
here's a list Fangraphs did in 2019, using 9million per WAR
looking at this, pick 12 would value at not even 2.5 WAR in 2019. The numbers would have changed since then, but not drastically so