When you go truly multi platform with UWP everything will be paid for with In App Purchases

In App Purchases (IAP) are how everything will be monetized in Windows 10.

Well, at least in part.
If you have any other thoughts I'd love to hear them.

In the Windows 10 every app will have a single store entry with a single price. Clearly this won't be appropriate when the app or game varies greatly on different platforms (device families).

Consider a game that exists on mobile, desktop and Xbox. Today you'd expect the gameplay to be very different across the platforms and also the price of the game to be different on each and you'll probably have to pay for each platform separately.

Obviously this won't work when there's a single price.
The solution then is for the app to be free and then you pay to unlock the app on each device you want to use it on.
There's an important UX consideration here around apps that appear to be free but then you have to pay for them once you've downloaded them.
A possible solution for this is to provide a free trial on each platform and then a paywall for the full game.

Obviously, some goodwill in the pricing will be nice and appreciated too.
For instance, if I buy the Xbox version first I might get the other platforms included too. Or, if I buy the mobile version first then I'd like a discount when I later buy the Xbox version.

Or maybe you use different monetization strategies for different platforms. Maybe the game is listed as free in the store but monetized with adverts in the mobile version and paid for (via IAP after a trial/sample) on the Xbox. With the desktop version being the same as one or other of those as is deemed appropriate.

Apps might be even more varied than games in their differences on different platforms. The near canonical example of this is the phone or tablet that is used for data collection and the desktop version that is used for reporting.
For some scenarios the above won't work or be appropriate and so having separate store entries for different platforms will be a better solution.

There are lots of unknowns here. The thing to note is that it's very unlikely that there will be a simple solution that works for all (or even most?) apps and games.

If you're building for Windows or Windows Phone and aren't yet familiar with IAP then now might be a good time to get prepared. You'll find documentation on MSDN.