One thing you can do instead of buying USB drives, is just buy a regular drive and put it in the USB case you have, you can save a bit or get a larger drive for the price. I paid $20 for a USB case and each time I put a larger drive in my laptop stick the older one in that case.
I still want to do some kind of offsite media other than the USB drive to supplement it. I found a tip on another site about making an archive folder and placing documents in there that are not used anymore but you want to keep around just in case like documents for previous jobs, old tax, bank, and other info. Then we will move stuff to that at the end of a year and back it to CD/DVD only then so it makes a bit less that I need to burn.
I've had pretty good luck with cd's not going bad, still have some backups from the 90's that read fine but I do plan on replacing the old every so often.