I have ALWAYS writen down our room cost for every trip we have taken with points. We purchased in 2000 and now own 627 points at BWV and BCV. I have always either called for prices or used web site and the amounts include the taxes that would have been charged at that time.
Our next WDW vacation is currently at BWV in a 2br and that would have cost us $6,457. This is for October.
Counting point usage thru the end of 2009 our current amount we would have spent if we had booked all of our vacations for cash (with out any discounts that may or may not have come available) would have been
$83,633
Last year we typed it all up and sent it to our guide, she was really impressed and wished she had thought to keep track of all her stays using her points!