They sell the USB drives at my local PC shop
the cheapest, and it is also a pen is £14 (approx $20)
and is 128MB. Plenty for all you plc progs
I have a 1.5 Gig USB drive (USB 2) (£70 - $100)
They are no bigger than a black marker pen top. The beauty of them is, you just plug them in and they are recognised and working within 2 seconds. You can also just plug them in someone elses computer and have access to all your files (ideal for updating everyones copy of the latest program update)
I keep everything I might need on that including all my software cd disks and windows XP disk and PDF manuals.
I once had my laptop fail and needed a full reinstall of the operating system and everything. I was on the job and miles from the office. I had to go back and spend the afternoon looking for all the disks and re-installing.