How Hard is it to Manage Different Unices?

vrmlguy asks: "Where I work has several Unix-based servers, all running the same vendor's OS. We are getting ready to buy another big server, and management wants to get bids from other vendors. However, our staff is only familar with our current vendor's OS. Yes, I know that any two flavors of Unix are more alike than not, and yes, I know about the Rosetta Stone for Unix that makes it easy to transfer skills. I want to know about the down-side: What's the difference in the cost of operations between a mono-culture and a shop running two or more vendors' OSs?"