Which map do you prefer to use on Locus?
There are several! How does it differ from the paid ones?
What do you want to buy?
It's just like in a bookshop: there are countless maps, and which one you choose depends on your needs, personal taste and budget.
In the Locus store, you can view the coverage area and previews for all maps, and read the details.
OpenStreetMap maps are free.
Personally, I mainly use openandromaps, which are based on OpenStreetMap, and LoMaps.