I would like to be able to automatically save a number (portfolio value) everyday.

Here's the sequence of steps that I'd have to go through to save the number:

1. Open the spreadsheet (if not open) at the same time every weekday.
2. Update prices with the following add-in. It's available free from Microsoft. Stock Quote Download
3. Enter the date and then enter the daily value.

I've attached an example spreadsheet. The prices won't show up unless the add-in is installed.