I think of it this way: Buying ARM for some billion dollars would give Apple the advantage of having unique processors on its devices. Is this enough justification for such an expenditure? I think it is not for a various reasons.

1) No matter how big the Apple is, the market would create new CPU architectures that would eat Apple's initial monopoly advantage over time. Investments are made for long term gains, so in the long run its not a good investment.

2) Apple can already distinguish itself from competition by its engineering, philosophy etc. a cpu architecture is not going to make enough difference. It would be wiser to spend money on innovation than some cpu architecture that already exists and is beneficial to Apple.

You can probably compare this to HP buying Palm for WebOS. HP bought it for having a strong software portfolio for its mobile devices.

The difference though is that ARM has to rely on its ecosystem (e.g. all other partners who bundle their products with ARM cpus) to survive. It needs to be licensed to the market. In case of WebOS, it was only used by Palm itself.