I think a spreadsheet is most useful (though not necessarily convenient while in the basement). I like to track my expenses (parts i've put into a machine, my time, etc), then subtract game plays, subtract general 10% depreciation, and get a true value of what I should sell a pin at (if I were to do so). That way even if I don't profit, I know I'm not getting screwed. Also when somebody goes to buy it, and they try to talk me down, I can pull it up and go "See all these costs, yea.. stop trying to lowball me"