Sometimes it is easy to put a dollar amount on the loss, sometimes not. Let me explain.
Well, I have lost plenty in land deals, forfeited income in start-ups, tech stocks, companies that eventually went bankrupt. I have not added them all up (do I want to?), but none of mine were as much as the OP's condo in a single bad deal (assuming it was paid with cash). Man, that hurts.
One expensive lesson, or numerous little less expensive ones? I am happy with the latter, I think. That gives me more stories to tell.
Like NW-Bound, I've had only small modest losses. ..
My biggest loss, which was the lost time in the failed startups, was substantial. I did not put my own money in the ventures (hey, I was not that big a fool), but as a founding member, contributed about 3 man-years. That number of hours was a conservative estimate and could be a lot higher, because when one was so busy, who was counting?
Compared to the alternative scenario where I did not quit megacorp job, do I count the loss as the gross income or the net after all taxes? Remember that it would be the net that I would have if I did not make that career move, not the gross. But then, there were the forfeited benefits, the 401k matching, etc...
But the above was my biggest loss, a 6-figure sum no matter how one looks at it. There were other losses that were a lot cleaner for accounting, such as $20K on a land deal in the late 80s, $10K on some stupid small-cap stocks in the early 90s.
But when it comes to my other losses when I became an active investor starting in 1999, it became fuzzy. Here are some examples. I had to install the defunct Microsoft Money program in order to look at my old portfolio file to pull out the following numbers, as sorted in order by that program.
AMD -$16,683
LU -$9,565
HLIT -$9,497
TER -$9,441
UYG $-9,057
CNXT $-8,499
MOS $-7,792
SCHW $-7,244
LVLT $-6,983
AAUK $-4,639
IRF $-4,528
VGK $-4,478
etc...
I shall stop now.

See what I said in another post about how diversified I have been. Still, these losses add up to a 6-figure sum, you say...
The point I will make now is that these losses were canceled out by my gains. Counting again from large to small, I see
USD $16,211
POT $14,586
WFR $11,381
APA $11,038
OIH $8,025
TRA $6,702
BBY $6,542
HUM $6,458
TXU $6,448
NE $6,118
DRYS $5,807
etc...
The above were all
realized gains and losses, incurred from 1998 up to when the Microsoft Money program went defunct a couple of years ago.
Now, I held individual stocks, so I could tell how much I gained or lost for each position. If you own these stocks inside an MF, or inside an index, how would you know?
WINR. I let a $600K unrealized gain dwindle to a worthless security.
That was an
unrealized gain, so your actual loss was only the stock basis. Still, that "lost opportunity" really hurt!
My 2 biggest financial mistakes were:
1- Buying a large chunk of ATT Wireless stock in 1999 on margin. My wife was working for ATT at the time and everyone was saying how this stock was going to be a "sure thing" with the way all those tech IPO's were skyrocketing in the late 90's. The stock never went much above the opening price and when the bubble burst I was forced to liquidate a bunch of my positions at the bottom of the market to make my margin call. The stock ended up in the toilet.
2- When the market tanked in 2008 but then quickly recovered, I got spooked by all the bad news and was convinced we were headed for a double dip, so I looked to find a good exit point, deciding to liquidate my portfolio once I got back close to even with what it was prior to the crash. When I hit the mark in late 2009, rumors were also flying around about Steve Jobs health, so not wanting to take any chances I liquidated a HUGE position that I had in AAPL which was trading at around 170. Shortly after I liquidated, Apple announced the release of the first iPad , and the rest is history. Today AAPL is trading at around 420. In the end this move cost me around a million dollars.
I actually remember your post announcing going to cash, and then recently your decision to buy back AAPL too. I wish you well.