The Post states the fact Beats Music had only 111K subs as of March may have affected the deal's price. "Apple hadn’t even begun its due diligence process when news of the number came out."
Billboard reported a few days ago attempts to negotiate a price for Beats Music (not fully owned by Beats Electronics) was one of the factors slowing down buyout talks, and that Apple was "nowhere near ready" to have the talks leaked to the media when they were.
The paper also reports Beats, which likely reaps high margins on sales of its pricey headphones and speakers, had 2013 revenue of $1.3B - that suggests Apple is paying just 2.3x sales.
Apple's WWDC conference starts in 5 days.