I'm literally making up numbers. I've never even seen a 1215, let alone rebuilt one. The little bit of looking I've done for other machines of similar size/horsepower goes something like this:
Rings $125 to $150. (It seems like around $40 to $50 bucks a cylinder or so)
Pistons: $75 to $100 apiece.
Cylinder sleeves, if needed: $75 to $100 per cylinder, if used and necessary. Boring/honing a non-sleeved block is probably comparable.
Bearings/bushings per rod: $40 to $50.
Main bearings: $20 to $30 each.
Gasket set: $100 to $150. Head gasket is around $40 to $50 bucks alone. Some/all the rest can be made with silicone, I expect.
Valves: $10 apiece.
To do everything, at the cheap end, if it were a Yanmar, in parts you would be in the engine roughly $750 or so in only parts, to do sleeves, valves, pistons, all bearings, etc.
http://sheaftractor.net/
As an investment it's not that bad, because the machine should be good for another several thousand hours or 30 years of use. When you break it down to cost per hour, though, say you use the thing for 750 hours (actually a lot more than it seems) you're in the thing a dollar per hour in parts amortization. If you paid $3000 for the machine, you're paying another $4 per hour for the machine in that time too. All told, less than $10 per hour for 750 hours of use, including tire wear, maintenance and so forth seems really good, plus, at the end of the time, you'll get much of that back.
You should check out parts availability for that machine. I'd suggest calling
these guys as a start and see what they have; then you'll also have real numbers, not something made up by a guy looking at Yanmar parts prices!

Let us know what you find out, too, so that we know and future people can figure out what they're up against.