The company that makes batteries for the electric fence dog collars
used in our pet containment system encases a standard, inexpensive 1.4
volt battery (retails for about $2) in a plastic "Power Cap" and then
charges the poor end user $14 each! How can I disassemble the plastic
piece and then replace the actual cell myself?

The power cap is made, because they don't want it to fall off the
dogs. And it's obious that they can make more money out of it.
I would suggest that you follow their plan.
But, it seems like you need to buy this over and over. So, to save
those money, you need to break it apart. That's the only way. The
"Power Cap" is assembled but it contains a part which doesn't allow it
to open. So, you have to get some instruments and break the cap gently
so you don't break the important parts. You should break the top part,
so that you can put it back together with some glue or something. Then
you should be able to change the battery.
But, if I were you, I wouldn't waste my time on this, but rather buy
them for $14 each.

What is the brand name of the collar/system?  Perhaps someone can find
you a cheaper source online for the "Power Cap" batteries.