For the Canadian "spin" on this question - we don't have a differentiation between short term and long term capital gains, but the importance is the distinction between whether it is a capital gain or business income. There are a number of criteria to look at to help determine whether a transaction is considered a capital gain or business income, and best that anyone in the situation review the rules in detail and consult a tax professional (individual facts are very important in figuring the right treatment).
If the conclusion is that it is a capital gain, Canada does have "personal use property" rules which can have implications. For example, is I have a small collection of baseball cards for personal enjopyment which I spent $200 on, and eventually sell the collection for $800 - the personal use property rules "bump" any proceeds or cost amount below $1,000, up to $1,000. So instead of having a $600 capital gain, proceeds of $1,000 and cost of $1,000 are reported and there is no capital gain (so no tax). There are even more specific "listed personal property" rules to be aware of depending on the type of items involved (don't appear to impact baseball cards, but could impact other collectibles).
If the conclusion is business income, I'd also just say - make sure to consider sales tax implications. I know these differ greatly between Canada and the US on implication/registration, etc.
That all being said, the "cost" aspect is straightforward and as noted, the cost base for the card (whether sold for a capital gain, or held as inventory and sold in a business), would be as noted - if you buy a 40 card pack for $4, the cost base for each card in the pack is 10 cents.