I have seen several from equipment vendors, but they rarely factor in downtime, troubleshooting, maintenance, etc. Of course our prototyping needs have always been pretty serious (larger parts & assemblies, 0.1mm general tol., finish to spec etc) If you find the right service (and do a lot of work) I think it comes out roughly the same when you factor employee expenses.
I think the largest advantages of owning your own machine have less direct to do with accounting, and more to do with flexibility, quality control, logistics and scheduling. Of course these factors eventually translate to meaningful numbers for accounting but sometimes take a bit longer to do so.
One has also to consider that when you have it in-house you can do the stuff which otherwise you would not have done because of costing, etc.. Another thing which you might consider is selling the extra time. In the end as caddit said it all boils down to accounts.
Being on the side of selling both systems and services here's the what I've noticed in the last 15 years. Those that start off outsourcing reach a certain point where they are doing a lot parts and/or spending a fair amount on outsourcing where they've justified purchasing a system. Often times once they get the system in-house thier return on investment is very short, sometimes months because they end up doing more parts and more iterations than they every thought they would need to do. And with larger companies, other divisions get in on the action and want to utilize the equipment or begin to outsource as they see first hand the benefit.
These customer tend to start outsourcing again because thier capacity keeps getting filled. They also outsource as they have needs for other materials that weren't capable on the system they purchased. Then as volumes increase again or they have a need for other materials they justify another system. So we see this cyclic trend.
Also with the rise of using these technologies such as FDM for manufacturing of end-use parts, many customers are not just building the prototypes of their designs, but using the parts for jigs, fixutre, assembly aids or even as the actual part in low volume applications. So by bringing in other departments, not just the design engineers, a system can be easily justified as mutliple departments can benefit from the equipment.
I can possibly help determine if you are looking at the FDM process. I represent the uPrint, which is manufactured by Stratasys. My company is a full line VAR representing the uPrint, Dimension and Fortus lines. CatalystEx, which is the printer driver that comes with these 3D Printers, easily can tell you the amount of materials used and print times. I would be willing to do a consultation with anyone who is interested in learning more about the FDM Process and figuring out the ROI. We have simple calculations that can determine the cost of models. We can even talk about consumables, maintenance, usage and applications, etc. Visit me online at www.uPrintSource.com or email vbarile@uPrintSource.com. There's a webcast on 9/10/09 1pm EST if your interested. Register on our website.