Spotrac does something called a Market Value. They say they explain it, but the link doesn't work for me.
https://www.spotrac.com/blog/how-mlb-market-values-are-determined/
But based on their Market Value pages:
(here's Aaron Nola's for reference:
https://www.spotrac.com/mlb/philadelphia-phillies/aaron-nola-17684/market-value/)
It looks like they find some comps, average those comps, then deviate from them in correlation to how the player whose value they are determining differs from those comps. For pitchers they appear to use GS, IP, k/9, bb/9, ERA, WHIP, FIP & WAR, & then make 1 more adjustment.
They come up with 6/$147m for Nola, so. not sure why they project he will sign for less than that.