You 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. A few months back, I proposed an experiment. It was a bogus mutual fund made up of four stocks, each bought in 1992 and sold exactly eight years later.

I chose EMC (NYSE: EMC), Clear Channel (NYSE: CCU), Xilinx (Nasdaq: XLNX), and AOL, which is now part of Time Warner (NYSE: TWX). But any number of former darlings would 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 eight short years. But there's a catch.

In those eight 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 fund's side. I thought you'd point out that nobody could pick just those four stocks, much less time the market so perfectly. 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 invest $1,000 a year for 20 years. If you earned a slightly more reasonable 20.02% per year, you'd be out a painful $80,000 in fees and lost profits.

And there's nothing random about this scenario. That 20.02% return is the figure 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. John Bogle -- the founder of Vanguard Funds, of all people -- makes the case 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 just 16% of your total returns ($385,000 out of $2.4 million) in my "outrageous" mutual fund example. That sounded like a lot to me, but apparently not to Bogle -- or to some others. In fact, for most of us, it will be worse.

For one thing, we won't be making 5,900% every eight years, as in my example. That's because for every Google (Nasdaq: GOOG) your skipper catches at IPO for a quick pop, he'll surf an XM Satellite Radio (Nasdaq: XMSR) for a hair-raising plunge. But mostly, we'll ride the likes of Alcoa (NYSE: AA) and Johnson & Johnson, 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, at the wrong times. That's why Bogle says you'll do worse than "average" -- 8.5% per year by his estimate. Plus, you won't invest for just eight 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 return of 8.5% per year, these "intermediation" costs can steal up to 80% of your rightful profits. You read that right. Not a mere 16% 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 important, 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 outlook for stocks. I think we'll do better. But even if we go back to my assumption that you manage the outlandish 20.02% per year that Stock Advisor members could have earned since the service started in 2002, you're still forking over $80,000 in intermediation costs every 20 years.

If you 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 to get started is a few 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. Johnson & Johnson is an Income Investor recommendation. Time Warner is a Stock Advisor recommendation. You can see all of David and Tom's recommendations instantly with your free trial. The Fool has a disclosure policy.