This is normal business practice. If you know, then the companys competitors know too. From the price they can infer all kinds of things. If you paid above market price maybe it is a strategic asset, if you paid below market price maybe it isn't central to the core strategy but too good to pass over. Etc.
It's normal business practice but that doesn't mean it's logical. The real reason in most cases is just that most people and businesses consider anything to do with money to be private.
This has some well-known negative side-effects. I've occasionally wondered what it would be like to work in an environment where internal transparency at least was the rule rather than the exception.
I suspect it would only work for organizations that were less than the non-exclusive Dunbar number in size.
In the organisation I work in we deliberately have an explicit transparency rule. We are quite small at this point, but it is very nice to be able to have open discussions around just about anything.
Richardo Semler's company Semco has been working for years on very open internal information flows with good results, in larger organisations than what we have. I recommend reading his book Maverick.
Anyone know why?