I have a mini a iphone and an ipad. Why cant i share music between these devices without pating money

You can sync music from a computer with iTunes to each device, but not device to device. If all of the music was purchased from the iTunes Store and the devices are logged in to the same Apple ID the music should sync. If the music was from other sources (ripped CDs, MP3s, etc) you must add that music to iTunes or the Music app on your computer, then sync your devices to that computer.

All of the above options are free. There is also a paid option that stores your music in iCloud and syncs it to your devices called iTunes Match. All of this is described here→Sync your iPhone, iPad, or iPod touch using your computer using USB - Apple Support