You guys never cease to amaze me.

Was I a little rough? Yes. Did I bash the mutual fund industry for robbing us blind? Sure. Did I expect some hate mail? Of course, but not this.

Feeling a little lost?
Don't worry -- here's a quick summary. Back in August, I proposed a little experiment. It was a bogus mutual fund made up of just four stocks, each bought in January 1990 and sold exactly 10 years later.

For my four stocks, I chose EMC (NYSE:EMC), Clear Channel (NYSE:CCU), Xilinx (NASDAQ:XLNX), and the obligatory AOL, which is now part of Time Warner (NYSE:TWX). But any number of former highfliers could have done the trick.

The idea was to show how your modest $10,000 investment could have ballooned to more than $2.4 million in 10 short years. But there's a catch.

In those 10 short years, you'd have paid your mutual fund manager some $85,000 in fees and surrendered nearly $300,000 more in lost profits (capital gains not earned on those fees). So, instead of $2.4 million, you'd be sitting on a lot less.

So, you hate me, right?
Of course you do, but I thought you'd take the funds' side. I thought you'd point out that nobody could pick just those four stocks, much less time the market so perfectly. In other words, I thought you'd say that my $385,000 blood money is a gross exaggeration and unfair.

So you can imagine how amazed I was by what most of you really said. It'll amaze you, too. But first, let's revisit my second hypothetical -- namely, that you invested $1,000 a year for 20 years. If you earned a more reasonable 22.8% per year, you'd be out a mind-blowing $90,000 in fees and lost profits.

And there's nothing random about this scenario. That 22.8% return is the figure that industry watchdog Mark Hulbert says David and Tom Gardner have delivered annually to their Motley Fool Stock Advisor subscribers. For more details, check out "Don't Invest Another Penny." But please come back, because this is where it gets good.

You got worked like a chump!
Or so you told me. Apparently, you don't mind my comparing Wall Street to the IRS on steroids. You took me to task for understating the case -- for trivializing the real cost to you as an investor, at least on a percentage basis.

And you're right. Of all people, John Bogle -- the founder of Vanguard Funds -- makes the case bluntly in his new book, The Battle for the Soul of Capitalism. Bogle shows how you don't need blowout returns (like in my superstock '90s example) to make the case against mutual funds ... you need time. Here's why.

Beware the "Tyranny of Compounding"
As it turns out, the scourge Bogle calls financial "intermediation" costs would have eaten up just 16% of your total returns ($385,000 out of $2.4 million) in my outrageous example. That sounded like a lot to me, but apparently not to Bogle -- and to some of you, either. In fact, for most of us, it will be worse.

For one thing, we won't be making 1,250% every 10 years as in my example. That's because for every Google (NASDAQ:GOOG) IPO your skipper catches for a quick 350% pop, he'll surf an XM Satellite (NASDAQ:XMSR) for a 50% one-year plunge. But mostly, we'll ride the AT&T (NYSE:T) tiger, right along with the rest of the widely held list.

And even if your manager does catch lightning, he'll buy and sell too often, and at the wrong times. That's one reason Bogle thinks you'll do worse than "average" -- 8.5% per year by his estimate. Plus, you won't invest for 10 years, but more likely 25, 30, or even 45 years or more. Think that's good news? Well, brace yourself, because this thing really gets ugly.

That'll be 80% off the top, sir
According to Bogle, if you invest for 45 years at his expected market return of 8.5% per year, these dastardly "intermediation" costs can steal up to 80% of your rightful profits. You read that right. Not a mere 28% like in my ridiculous little scenario, but up to a full 80%. Ouch.

For one thing, Bogle uses a more aggressive 2.5% for intermediation costs. That's because he goes beyond reported "management fees" and includes taxes, transactions, and timing costs. And given that Bogle founded Vanguard, the most trusted mutual fund company in the world, I'm inclined to believe him.

More importantly, Bogle realizes that the more realistic your returns, the more deadly that 2.5% becomes, especially when compounded over the years. In other words, costs kill when your portfolio keeps doubling every six months, but when it's doubling every 10 years or so -- costs kill you dead!

What you can do about it
Frankly, I don't share Bogle's "realistic" outlook for stocks. I think we'll do better. But even if we go back to my assumption that you manage the remarkable 22.8% per year that Stock Advisor members could have earned since the service started in 2002, you're still forking over $90,000 in intermediation costs every 20 years.

If you sort of resent that, here's a solution a lot of folks are considering. Start managing your own investments. Of course, you don't have to jump in all at once, and you don't have to dump all your funds -- right away. But you can see how important it is that you give it some thought -- and soon.

All you need is great stocks. Give Stock Advisor some thought. You get the top picks each month from Motley Fool co-founders David and Tom Gardner, and you can try it free for 30 days. There's no pressure to subscribe -- and if you do decide to join after your trial, it sure as heck won't cost you $100,000. To learn more about this special free trial, click here.

This article was originally published Sept. 29, 2006. It has been updated.

Paul Elliott owns shares of no company mentioned. Time Warner is a Stock Advisor recommendation. AT&T is a former Stock Advisor pick. XM is a former Rule Breakers pick. You can see all of David and Tom's recommendations instantly with your free trial. The Fool has a disclosure policy.