Warren Buffett says low-cost investments are essential for the average investor - here's how much it costs to use 8 common investing apps

• Warren Buffett has long championed low-cost investing, often recommending Vanguard's S&P 500 index fund for the average investor.
• New fintech - financial technology - companies are attracting users for their convenience and sleek interfaces, but the fees associated vary between apps.
• We looked at the fees and the expense ratios for eight fintech investing companies on the Forbes Fintech 50 2018 list.

Investing can be a difficult process.

Deciding what to invest in can take hours of research - but before you get to that point, you have to decide where you plan to invest.

Billionaire and legendary investor Warren Buffett has long championed Vanguard's low-cost investment options. Vanguard primarily offers index funds - passive investments that allow ordinary people to invest in the stock market with little effort and incredibly low fees.

"My regular recommendation has been a low-cost S&P 500 index fund," Buffett said. "To their credit, my friends who possess only modest means have usually followed my suggestion."

Investing in a Vanguard S&P 500 index fund can be done directly through the investment giant, which manages $4.5 trillion. Opening an account at Vanguard is free.

But many investors are turning to financial technology companies - fintech for short - for their convenience and sleek interfaces.

However, as Buffett emphasizes, one of the most important factors when choosing an investment platform is fees. We looked at the fees and the expense ratios for eight fintech investing companies on the Forbes Fintech 50 2018 list.

Two of the apps - Robinhood and CircleUp - are free for most investors. The rest charge a percentage or flat fee, typically based on the amount of money in your account. Several apps also offer multiple levels of service, like a premium account, which comes with a higher fee.

Many of the investing apps offer ETFs - or exchange-traded funds - a low-cost passive investment product similar to an index fund. Even though ETFs are passive investments, they still have expenses to cover. The expense ratio is the fee charged by the ETF, which is in addition to the fee charged by the investing app.

Fintech companies that offer real estate or other types of investments are all over the place in how much they charge. Minimum requirements to open an account may also be an important factor for people with smaller investment plans.

Below, compare the costs for each of the eight investing apps that made the Forbes Fintech 50 2018 list.