This is really timely for me.
I just did a pottery fair. I have no way to really account or the clay and glaze I already had on hand, and I have not metered the electricity. Nor did i factor in the cost of kiln, brushes, tools, etc already in my posession. So my "expenses" include only materials bought after deciding to do craft fairs. After subtracting my expenses, I made around $80. I have a lot of pottery left on hand for future fairs. I also have barrette backs, pins, ornament hangers, screw eyes, clay and glaze. I made a rule for myself that I would bank 1/2 my "profit" and use the other half as operating money.
Then I got an e-mail that glaze is 25% off this weekend. 25%! That's a good deal!
Except I don't *need* glaze right now. What I need is a different type of clay that will improve my sucess rate (less breakage) with a particular type of pot that sold. The clay is $20. Which leaves me $20 to bank against future needs, not spend on two jars of cool glaze. I have glaze.
And yet, I just want to go buy a whole bunch now while it's "cheap". (because there will never be another sale on glaze? Because I will run out?). Because it's ON SALE!