$39.99 seems about right... One device at a time.
I am assuming you mean platform? App store won't differentiate the device, just the Apple ID. I think that at $39.99 per platform/OS you are pretty spot on as to what is fair. However, just who out there really is using Office only on iOS? There is going to be at least one desktop platform out there and that is then $80. You can get the premium subscription for less than that, and you also get the cloud storage, convenience, updates for as long as you subscribe, etc.
Then of course, everyone here forgets that they have signed their ownership away by agreeing to use the App store. All those apps you buy with a 1 time payment, you don't own them. The developer AND Apple reserve the right to pull those apps from the App Store at any time - so if you need to reinstall and it is no longer available, tough!
With App stores on all platforms it is the same, goes for Android, OS X, Windows, Windows Phone, etc. We've already transitioned from owning physical copies of software, we've already given up that right, without most of us ever knowing it. The central repositories for app distribution are in themselves services. We're all operating like Linux has for a long time, but we are paying for access to these repository services through our OS license and/or hardware purchase. Then we pay one time fees to software developers only for access to their apps during the finite period that they are available in said repository.
Software as a service is actually a win win model for all parties. For the consumer, you gain access to that app on all platforms you use, and you also gain access to the latest versions without having to reach back into your pocket to pay another "1 time" fee, etc. Companies and developers gain a more steady income stream, which simplifies their accounting as well as they don't have to defer income.
The point is you've already given up outright ownership of your software, that's just not an argument that is valid anymore.