You ca do a cost analysis all day but the minute you go by it and select your unit based on it, it will turn out different. Just Murphy's law and all a gamble. You could get a used unit that goes to******the next day and you have no backing for it. You can also buy a new one that ends up being a lemon and you will have trouble from day one. It will be covered but will be a thorn in your side and you will not be able to enjoy your trip.
I think it really comes down to personal preference. We prefer to buy new. Yes we end up spending more than what others spend on used units. But we prefer to have less worries in our mind that something is going to go wrong with it. With an older unit I will probably overlook something that is wrong or a potential problem. And with our limited schedule on camping, I don't want to have to worry about a trip being ruined. We also have the warranties and like the peace of mind. Another reason for not buying used for us is odors and fabrics. All the used units we have been in, stunk. I don't like used mattresses, but those can be replaced. Also, hopefully you will adopt the pyramid I your black tank.
Now if I were retired and had time, the I would probably have a different mindset.