It doesn't even matter. Don't you get it? They already buy 220 ppi 15.4 panels for $160 a piece (estimated). You don't have to be a genius to see that cutting same panel to 27'' size would cost only about 3 times more, not 10 or 20 times more.
Edit: For 5120x2880 iMac, they wouldn't be able to use same 220 ppi panels they use for MacBook Pro since they need panels with a little bit lower pixel density. About 218 ppi.
When you start considering yield problems, 10x is a real possibility. It's much harder to make a perfect 27" than a 15". Triple the surface area is triple the chance for error.