If your willing to pay for an excellent model that will answer your question and more both Conklin de Decker (includes helicopter cost database) and SLM (in my opinion, has a better model) offer solutions. Both consider residual value.
Conklin is at:
http://www.conklindd.com/cgi-bin/sof...87d+1107062294
SLM is at:
http://www.aircraftcostanalysis.com/
I have used both but have not stake in either so I hope I'm not breaking any forum rules.