No two people are worth 3.2 BILLION dollars (unless they are people who already have 3.2 BILLION dollars.)
Dre and Iovine are worth $1.5 billion net worth combined.
But seriously, this is not how deals are valued. I'm a finance analyst for a $5 billion software company (Intuit). We look at strategic acquisitions all the time and like Apple they are typically a number of smaller acquisitions as a play for key technologies that together in our stable have more value than being apart.
That's partly what this is about. But a nuance to this is that Beats/Dre/Iovine may not be worth $3.2 billion in, say, Microsoft's stable because Microsoft may or may not feel it has the moving pieces to make it worth taking them off the table from everyone else at such a price.
If Apple does, it's because their M&A analysts and consultants have identified that future returns on that investment are substantially in excess of $3.2 billion but only because of what they can do with it. What company XYZ can do with it is completely immaterial to Apple's valuation except as baseline for knowing how much over that do they have to be to guarantee that Beats will take their offer and not XYZ's.... especially if there's a chance that XYZ can throw out a similar number.
As Warren Buffett is known for saying, "Price is what you pay. Value is what you get." $3.2 billion is the price they may be floating, but the total value goes beyond just tangible book value of the assets involved or the current operating cash flows and includes, generally, an analysis of the future operating cash flows discounted to net present value.
Is it an exact science? Of course not. But such valuations usually are approached from three or four different methodologies and they take a best, worst and average case scenario of them to arrive at an offer based on whatever metrics make the most sense.
It's not dart throwing when there's this much money involved, unless it's a financial institution you're buying in which case there aren't any widgets to count and the "value" of a good chunk of the cash generating assets is highly questionable.