It's hard to know what market rate is these days.
I've said before that Keuchel is not a good comp, but Porcello with a QO attached to any signing on the open market may not be valued as much as some of us think he is worth.
There are some similarities with Keuchel. Keuchel had an "outlier year" in 2015, and Porcello had one in 2016.
2014-2018:
Keuchel 67-45 3.28
121 ERA+
1.18 WHIP
4.35 FIP
3.11 K/BB
145 GS/950 IP
Porcello
74-56 4.04
107 ERA+
1.28 WHIP
3.95 FIP
4.04 K/BB
159 GS/994 IP
Keuchel is almost exactly a year older than Porcello, but Porcello will be his same age when they reached free agency.
I think Porcello is better, but how much better?
Other recent signings:
2019
$140M/6 Patrick Corbin (2 years younger)
$68M/4 Nathan Eovaldi (I feel Porcello is better than NE)
$34M/2 JA Happ (6 years older)
2018
$126M/6 Darvish (Close to same age when a FA)
$75M/3 Arrieta (About a year older when a FA)
$57M/4 Cobb (About a year under when a FA)
$38M/3 Chatwood (About 2 years younger)
$28M/3 MMinor (About a year younger)
Hard call.