$30 and even less is not uncommon for an older, slightly scuffed up, used phone. Or maybe you already have one, laying far back in your desk's drawer. Not a kind of phone most people would use as a daily driver. But as a dedicated bank-app thing - sure, why not.

Just don't connect it to untrusted wifi, disable bluetooth and all that. Chances are - that phone had its last OS update years ago, and could be pretty easily pwned [0]

[0] https://gist.github.com/jesux/64cf037c55c0d42196762c0ccacc73...