RE: 2006 Stage 3
See what others are selling for (ebay, dealers and online sites like autotrader help alot when searching) and check fnsweet.com for more info if you need.
There isn't really a 'set value' for Roush's. We're working on getting into the NADA, but the real determining factor is the current market.
1998 Roush: M112'd Aluminum block 4.6L 2V 6 speed track car
1999 SVT: Mostly stockish
1970 Mach 1: Resto-modded monster
2000 Roush #5004 - World's FIRST M112'd V6 Mustang. Sold, but not forgotten.