Obviously in terms of total cash paid out it is more expensive in gross dollars to have a maintenance plan.
If this statement was false then Xerox, and all other brands, and independent dealers would be out of business and wouldn't offer maintenance plans.
Hands down if you can service your own equipment or have an in house copier tech then it will be less expensive than paying click charges.
Machines often run longer than their maintenance contracts so there can be years of extra benefit by running your own maintenance.
The other responders are not having a clear indication of which is the cheaper alternative because they are looking solely at Xerox.
ANY OTHER BRAND, out there has a huge surplus of parts, supplies, technician manuals, and thousands of independent dealers willing to sell a part and offer service competition. Xerox parts and supplies are more expensive and harder to get because they make it that way. Other companies don't.
Bottom line is if you can afford a technician or know how to fix your own equipment it will be cheaper. If you don't want to bear that burden or don't know how to, then you should do the maintenance plan; but you are paying more in gross dollars for the luxury of it.