actually, I wouldn't bet that Cameron knew what was going on. Deniability is a key word for politicians: staff deliberately keep them in the dark about messy details so they can truthfully say, if push comes to shove, that they didn't know.
All Cameron will have needed to know about dinner guests is that they were wealthy Conservatives who gave money to the party. That's not improper in itself. In fact, I'd expect a Tory PM to dine with rich Tories. If he's held responsible for any of this it will be in his position as party boss rather than personal responsibility; the latter almost certainly will never be proved.
The issue here is more to do with the mercenary approach: not "The PM always likes to meet supporters" but "Give me a quarter of a million and I'll give you dinner with the PM". It gives the impression - as does the budget - that the government's intention is make the rich richer and ignore those who aren't. This is at odds with their claims that we're all in it together.