Yes, there is such device to increase your iPhone or iPad storage and it’s not just to save money. With a usb flash drive you can transfer files between your devices without using iTunes and also have a copy of all your data as a backup in case you need it at anytime! So you can have your big gigabit presentation file on your desktop and open it anytime without even an internet service.
Usb flash storage is even cheaper than upgrading your phone for the purpose of storage increase! 64gb costs around 60$