$4 just to add a widget to your N2 lockscreen?
Have you ever programmed software?
If yes, you should be able to appreciate the amount of work that is required not only to create an application, but support it. Develop doesn't stop with the first version. Bugs are fixed, new features are added, new OS's are supported, etc.
If no, read above for the reasons why.
I always found it odd that the Apple App Store was so filled with $0.99 applications. Most of the applications are way under-priced, but the culture was created were users started ignoring anything higher.
It's rare, compared to the Apple App Store, to find $0.99 applications in the Play Store. I find Android developers are more responsive to bugs and requests because they aren't focused on spitting out a ton of $0.99 applications. They are focused on the few properly priced applications they make. Of course that's a generalization, but that's been my experiences.
Also, it's only $3, not $4 and I find that to be well priced for this application. This isn't just an add-on that goes on your lockscreen. WidgetLocker is lockscreen that the developer created.