Castillo would MAYBE get exactly what he has right now, if he were a FA. Blake Snell threw 104 innings last year and put up 0.8 more fWAR than Castillo did throwing 175.1.
Blake Snell is unquestionably the better pitcher right now. His stats have been consistent. Nothing has dipped, in fact his velocity went up last year. Castillo on the other hand has lost movement, velocity, and strikeouts. If you squint you can maybe see a path where Castillo could get $25M, maybe $26M, because of what Severino got, though there's probably a tax on Severino's deal for having to play in a minor league park. So Castillo may not truly be underwater, but he's only breathing from the tip of his nose.