I'd like to use that data for a simple yet admittedly selfish purpose.
I want to find the cheapest prices before I go shopping.
Is it professional/ethical to do this? On one hand, the prices
themselves are definitely public and anyone can find them by walking
into the outlets. On the other hand, the aggregate data takes a lot of
effort to collect and is therefore valuable.
In my opinion, it's neither professional nor ethical to use information gathered and paid for by the company for your personal use, unless you first have their permission.
Databases and their contents should be considered company property. Just because you are in a position technically to have access to the contents doesn't mean you are free to use it for non-company uses. And the nature of the contents and how the contents were acquired doesn't change that fact. You generally don't have permission to use company property unless it is granted to you.
If the company publishes the information on a publicly-available website, then that implies permission (at least for using the data through the website). On the other hand, if the company sells that information, then that implies that you don't have permission.
If the company tells employees (either in written or verbal form), that they can use the data, then you have permission.
Otherwise, the best way to know for sure is to ask for permission. Talk to your boss first, and if she/he doesn't know, you could ask who could give you permission.
Something like "Hey, boss. The prices in the XYZ database - they would be handy for personal use. Is that something the company would permit?" is a reasonable question. It's also possible that this simple question would result in an answer like "Sure. Many of the employees use it that way." But you will only know by asking.
If I were the manager, I wouldn't think less of someone for asking such a question, even if the answer were "No. That isn't permitted."
I'd think far less of someone who just decided to help themselves to the data without asking first. I'd wonder what else they were helping themselves to without permission.