I find in web this adress https://apps.ubuntu.com/cat/applications/saucy/youtube-to-mp3/ and there show when program is free, but in ubuntu software center i have to buy it. why?
I'd recommend the easily usable and amazing
youtube-dl. You can get it from here.
After installing, you can get the audio in mp3 format by using the
youtube-dl -x https://www.youtube.com/watch?v=7ty7mk3UOmo
The way the Ubuntu Software Center handles free but proprietary (i.e. non Open Source) applications may be a bit confusing. However after we proceed to the download we will see the following:
So the price we have to pay for this application will be $ 0.00 which to my understanding is free as in terms of costs.