For the Stage 2, which is just an appearance package for the most part, should be around $15K above the Ford sticker price. Stage 3 is about $25K or so. It also depends on which Roush options it has too. You can almost guarantee most dealers will tack on a markup too like with Shelbys, RTRs, etc.
I bought my 2019 Stage 2 with dealer-installed supercharger used for $48K and it had 800 miles on it.
Yeah mine was $50k new out the door. Stage 3's are 75-85k but I've seen hold overs in the high $50's. I wouldn't pay more than $50k for stage 2 or low 60's for Stage 3. Unless you just have money to burn