these are already out there. if you don't own the lock already you aren't going to save any money.. if that's the point 
that lock must be $50, plus you need an arduino at $30. plus misc stuff, plus time. Is yours going to do something different or better? Or just a cool project to play with.
I think it would be much cooler if you use some kind of manikin (sp?) hand that actually pushed the buttons when you got close. That would creep somebody right out