The market I'd really think that would benefit from this (at least early on) is University students. Obviously, it isn't only limited to this, but in general uni students are generally making smaller purchases relatively often.
It is good that you have a narrow focus. What specific problem do university students have that is solved by your product? Also, have you thought about how you want to iterate with a small group of users?
The problem is that often, a lot of University students are extremely unaware of how much they spend. I would argue more so than people that are more experienced handling their expenses. The amount of my friends that have said to me 'Man, I have no idea where my money goes, I don't but anything, but it just goes!' is crazy. If I can empower Uni students to make better financial decisions through a better understanding of their expenses.
Changing behavior is difficult. That's the reason services like mint rely on bank/credit card transactions to get the data. You may want to quickly start testing with a few university students before investing more development time.