When it comes to investing, keeping costs low is the name of the game. That's because management fees and annual expenses can take a big bite out of your money. In fact, paying just 1% more in annual fees can mean you'll have $80,000 less in retirement!

Clark has long advocated investing in index funds over individual stock picks. Fortunately, there are a lot of services out there that will let you get invested in index funds and ETFs -- the latter being one of Clark's favorite kinds of investments -- for $100 or less.

Some even have no minimum at all so you can get started investing today with (literally) mere pocket change!

Read more: How much you need to save to be a millionaire by age 65

Investing with less than $5

3 ways to buy stocks for free or very cheap

Robin Hood

Minimum investment: None

Costs and fees: None

Robin Hood is the latest to try zero-commission stock trading by offering free trades from your smartphone (both iPhone and Android).

How does Robin Hood do this for free to you? They make money one of three ways: On the money you have on deposit with them in your online brokerage account; if you borrow (leverage) to trade; and by moving orders through particular platforms.

Betterment

Minimum investment: None

Costs and fees: .35% annually for account balances below $10,000

Betterment is an automated investing service that provides optimized investment returns for individual, IRA, Roth IRA & rollover 401(k) accounts.

Over 175,000 customers invest more than $5 billion with Betterment.

Folio

Minimum investment: None

Costs and fees: $4 per trade or $29/month for unlimited trades

Folio Investing is an online brokerage offering financial resources and investment products such as stocks, mutual funds and ETFs.

Just beware that if you make fewer than four trades a quarter, you  may be subject to a $15 fee.

Investing with $5

3 ways to buy stocks for free or very cheap

Stash

Minimum investment: $5

Costs and fees: $1/month to get started

With Stash, you can buy ETFs for as little as $5 because you're buying fractional shares. That's setting the bar for entry really low when you consider some ETFs can have a minimum investment of $1,000!

Stash makes its money by charging $1 a month for balances under $5,000. So if you truly are a small investor, your money will get eaten up pretty quickly. For those with balances of $5,000 or more, Stash only charges 0.25%.

That's competitive, but nothing to write home about; it's possible to pay as little as .04% to .05% so consider carefully before jumping at this one.

Acorns

Minimum investment: $5

Costs and fees: $1/month to get started

Acorns lets you invest small dribs and drabs of change from larger purchases. Here's how it works: You give Acorns your log-in info for your credit or debit card accounts, and then the app rounds up every time you make a purchase so they can invest your small change for you over time.

So let's say you buy something for $3.24. Acorns will automatically round the purchase up to $4. Then they take that extra 76 cents and put it in savings. Every time you hit the $5 threshold in your account, they take it and invest it for you.

How does Acorns make its money? There is an annual fee of $12 to use the service. At $12 a year, the reality is your money may go backwards in the early years. But if you stick with Acorns for years to come, it might be a good way to turn your spare change into a little nest egg.

And just like Stash, Acorns only charges 0.25% for those with balances of $5,000 or more.

Investing with $10

Apps to start investing from your smartphone

Loyal3

Minimum investment: $10

Costs and fees: None

With Loyal3, it's free to open an account and you can buy fractional shares starting at just $10 a pop. There are no fees to buy a stock or to sell it. Plus, the company offers direct access to IPOs (initial public offerings -- aka buying a stock before it first becomes publicly traded) starting at $100.

You only have access to a universe of 60 or so stocks, but that includes very popular companies like Alphabet (Google), Facebook, Tesla, Netflix, Starbucks, McDonald's and Coca-Cola, among others. There is no access to ETFs or index funds through this service.

How does Loyal 3 make money? Companies will pay them for increasing brand engagement through their social network and to oversee their IPOs and follow-on offerings. In fact, Loyal 3 recently partnered with T-Mobile to offer free shares of stock to millions of wireless customers!

Investing with $100

Schwab

Minimum investment: $10

Costs and fees: Varies from .09% to .29%

Charles Schwab is one of the nation's leading low-cost investment houses. They have several index funds that would appeal to investors just starting out.

A few examples include the Schwab S&P 500 index that tracks the total return of the S&P 500 by investing in leading U.S. publicly traded companies from a broad range of industries, and the Schwab Total Stock Market that tracks the total return of the Dow Jones U.S. Total Stock Market Index.

Read more: Clark's investment guide

Robo-investing: An inexpensive way to start building wealth