Like I said, the boxes are much better, but per disc price is about a third of that. Five sheets sure doessn't last very long, you KNOW you will be using twenty or thirty lickity split.
Now, if you go to a autobody paint and supplies store you will find that they will usually have discs in a box where you can buy just a few. These will be a little more than by the large box but WAY cheaper than the retail packs you are use to buying.
You REALLY need to go to an autobody paint and supply store if you are going to be doing any amount of this stuff at all. You get much better info, way more products and you will learn much faster.
I don't know about the 3M products you are using but I have seen some of those retail packs (I use to sell them) that were JUNK. I mean they put the worse junk in those packs.
Move on up to the good stuff you will never be sorry.