You also need to force them to justify their requirements, since asking for something way beyond what you actually need is an easy way to hide the fact that they don't understand what they actually need.

In my experience, people like that asking for 10x the actual requirement is fairly usual. But, every once in a while you hear someone say "we should buy the best, so we don't have to worry about it in the future" (when I heard it, that was a 500x cost difference).