BASS is free for non-commercial use. If you are a non-commercial entity (eg. an individual) and you are not charging for your product, and the product has no other commercial purpose, then you can use BASS in it for free.
The thing is, I read in the web, that if I decode mp3 I have to purchase a licence.
A license is needed for commercial (i.e., revenue-generating) use of mp3/mp3PRO in broadcast systems, streaming applications, other content distribution systems or for use of mp3/mp3PRO on physical media. However, no license is needed for private, non-commercial activities, not generating revenue or other consideration of any kind or for entities with associated annual gross revenue less than US$ 100 000.00. (link)