A little project of mine

[GAMEchief]

@Riddle - I think approximating would be more difficult than getting the actual numbers. And Indeed the most recent trends are more significant, however I'd like to feed as much data as possible to the programmer.

I think he's just talking about compressing the data, instead of storing every individual day if you don't need every individual day.
{
total: 500,
days: 5,
lastDateLogged: howeveryouwanttostorethissothatyoudon'taccidentall ylogthesamedaytwice
}

Average per day: 500 / 5

You can even store average instead of total for larger numbers/days:

{
average: 1000000,
days: 365,
lastDate: etc
}

Adding a new day to this average:
average = (average * days + newDaysValue) / (days + 1);
days++;
lastDate = newEtc;

Saves dem bytes, if you aren't using then anyway.
But I can see how knowing them would be useful (day of the week stats, or progress over time, since an item being worth 10g a year ago and 1m gold today isn't only worth 500k gold on average, but instead the rate of change trend is more important).
Calculating a [curved] line of best fit for a set of data would be insanely useful, especially if you can get the derivative of that line. It'd theoretically create the best predictor of which items are the best to hoard.