A couple of tips, if you lose one, you can buy 5 replacements for $1 (like $3.50 with shipping) here.
And for those of you struggling to get it back into the cube, practice this video for an hour or two, and you'll get it down. Make sure you alternate wrapping to keep the grid formation, as they suggest!
I was looking at the buckyballs company site (the one listed on the insert), where 10 spares cost a buck, plus 4 bucks for shipping. So 50 cents per if you go that way, vs. 72 cents apiece from that vat19 site.
I would say that having 10 spares might be a bit over the top, but I took them out of the box for the first time this evening, and before 10 minutes had gone by I'd lost one somewhere in my office. At this rate, I'll be using up my spare set, one piece at a time!