Currently Ubuntu Touch is in development and many of these details are not yet worked out.
Will the app ecosystem only allow free open source applications?
Can you make your app a paid app?
At http://developer.ubuntu.com/publish/ #4 states:
Choose your price
Not all software is free in the Software Centre. If you are charging
for your app, you’ll need to provide details of your Paypal account, a
phone number and a postal address. The minimum price you can charge is
Note: for mobile apps in the Software Store Beta, payments are not yet enabled, so stay tuned for updates on when you can start charging
for your app.
And also show a graph of the fees you need to pay if you choose to charge:
The Note is important because it refers to Software Store Beta which is currently what you can use to get apps to the Ubuntu Touch Developer Preview. However payments are not yet in place.
Can you place ads in the app?
This question is probably the hardest to answer at the moment, because at the moment it seems there is no "ad system". This would be important for many reasons:
It would be similar to spyware if a individual application would access and deliver information for placing ads, as well as resource consuming if many apps were doing it at once.
Need of approved ads, ones that would not hack/crash/do any buffer-overflow/automatically install programs/ect. or mislead to "Download Here" when a real download link is elsewhere.
Acceptable ad area, so we don't encourage targeting accidental clicks or guerrilla tactics that only frustrate users (are they really going to buy/download a product they were practically forced to click on). Possible area for banner format ads when you look at the design of many of the current apps:
Note: Just a mock-up and not actual ad placement or views of developers, nor am I saying there should be ads in the clock app.
There are many discussions about how it should be, but I think many free apps that are currently are for Android show how it shouldn't be: Requiring every kind of permission (and they say that is only to serve ads), pop up videos, slide in graphics, Begging for ratings, you name it - Will Ubuntu Touch be the same "anything goes" in this aspect?
Or will there be no ads allowed altogether - In favor of trial (of which can be added to the software center) and paid versions, or altogether free.
Possibly a framework for ads could made as hbdgaf suggested in chat :
So it was said it isn't ad driven. It should be. Not putting ads in
lenses, but a way to get something google-esque with an ad framework
integrated in to the app framework. Say I want to make a cool app for
u-touch and not get paid for it. Ad funding should be an option. Maybe
it still is, but ad is a hot-word so they left it alone and said it's
not a thing. Just saying... A framework should be in place. It isn't.
This is a way to get there from here.
For now it seems in the developer preview they are focusing on the "Core" apps and experience, and we have not yet seen how the paid/ad system will look.