Cap and Trade: The Punctuation Market
A friend of mine just exceeded her lifetime allotment of exclamation marks. She can try purchase more punctuation credits, but exclamation marks are in high demand, especially during the holidays.
Comma credits are volatile, due to the competing forces of texters (who avoid most forms of punctuation) and comma splicers. (This latter force also keeps the cost of semi-colon credits extremely low; frugal consumers make ample use of these under-valued symbols.)
There's a glut on the market for apostrophe credits, thanks again to under-use by the texting community.
If you want a real bargain, try buying bracket or caret credits.