I don't know what percentage of backers are the kind that do it merely to "support" a project, but I would guess it's not more than 10-15% of all backers for a given project.
The rest want some value for their money beyond feeling good about backing. You state you chose $50 for a license because that is what you think it is worth. I think you'll find out that what you think people will pay you for something and what they are willing to pay differs by a factor of 2-3.
So most of the people that want what you are going to make, even a little, probably don't want to spend more than $20 or $25. The difference between that and $50 is made up from goodwill, but that's really asking a lot of people.
I would recommend making some more tiers between $5 and $50, where everything above $20 gets a license. This way you capture the people that just want the product, and give others the ability to show their goodwill as well.
This isn't the kind of project that really needs swag. Nobody really wants a t-shirt. If you do something like promise to list backers (and their associated tiers) in the "credits" of the app, that would probably be great for most people.
edit: typos, and wording