You will also find that the number of pins you have to trade when dealing with the CM's is irrelevant. If however, you want to trade with others (non-cm's) it might be more of a factor.
While I am not a HUGE pin trader, I have a few that I like (and don't like). I end up buying the "Mystery Packs" trying to get some "Special" ones. Often times (of course) I am not so lucky. Either way, it's very easy to get into with just a few pins. As your "Kiddos" start trading for what they want, you will find that you need to buy more, if for no other reason to give them something to trade with, because the ones they have already traded tend to become the ones they want to keep.