I'm not saying this will work for everybody here, but I have just recently converted a 1972 Challenger to a 1970...I swapped the headlight buckets, grille and bezels(Most expensive thing), side markers, and taillight panel/lights for under $1000.00 total cash outlay in parts alone...
This was because I had a '69 Charger grille to swap for the Chally grille, and I got the rest at "Scratch and dent" deals..As a person who can do my own Engine/trans/bodywork, I feel I don't need to have the best panels as I can "massage" 'em to work....For the average Joe who has to pay someone to swap all of the 1971 Cuda specific parts to a '73, I still say one could do this for less than $3500.00, depending on how many smokin' deals they get, and if they are willing to buy a grille in need of work that nobody else wanted, and fix it themselves...