How will you update the prices? Manually? If that's the case I predict you'll get bored after a while. Also, it would not bring much value to your portfolio, if you wanted to use it to showcase your front-end skills then that's different.
For a back-end project, you want something that, ideally:
- is unique and interesting to you and potentially others, cannot be something generic that the recruting manager saw 50 times already (many people put stuff that they "created" while following some tutorials, that will not impress anybody)
- is an app/service that will work with minimal user input, it could periodically hit some API, scrape the web or use some other way to obtain data. Let's say that you want to apply for a job at a company that deals with IoT - then invest £30 in Raspberry Pi and some sensors, hook it up to your service using MQTT (so you can put another keyword on your CV
) and process it in some way.
- implements security in some, even basic, way
- is live and can be used straight away and is easy to use. If it's just an API then is has to have documentation (you might find Swagger useful)
- shows that you can write reasonably clean code, OOP, SOLID, KISS, DRY and know at least some design patterns, data structures and Java ecosystem, 9/10 times nobody will look at your code but if they will that means that they're already interested and you don't want them to have a heart attack
Bonus tip: when you start sending your CV out, in each one add a unique ref key to the link to your portfolio/app. Process it on the backend and save the stats so you can see which companies are interested. Not only useful but if they notice then at least it will make them chuckle and they'll remember you