They expect it to be a free service because there are other free services that do something similar.
In order to get income you can either
1) Get people to understand that your product is substantially better (for their actual needs) than the free competition. You will then be able to charge money for your service.
2) Find out how your free competition is getting income and copy them (ads?)

If you do in fact have features that are better than the free competition, I would recommend making two versions of your service:
A) A free version which has basic features that are shared with the free competition
B) A paid version which also has the 'pro' features which your free competition does not have. This could for instance be an ability to better customize the types of alerts you can get, or change the sound of audio alerts, etc.
C) (optional) It will also help you get sign ups for your paid service if you add a third option which is a little more expensive than option 'B'. Simply the presence of a more expensive option will make people see option 'B' as a better deal, than if there was no more expensive option 'C' (it's a psychological thing). Option 'C' should be everything you get with 'B', but also comes with something extra. This something extra could for instance be an email that the user gets once a week summarizing 4 - 5 news stories about the stock market, or anything else you can think of. In the end though, this is not about 'tricking' anyone into paying for a product they don't want, it's about helping them make the decision to get something they will be happy with. In other words, your product (option 'B') should be fairly priced for the value they will get out of it.

Keep in mind, that even if your product doesn't have any obvious abilities that your free competition has, if you package your product in a more easy to access, and pleasing to use way (i.e. make signing up easier, make the UI more intuitive and pleasing to look at, etc), many people might be willing to pay for that too, even though you might not see it as a feature.