Investing in bitcoin: Tony Robbins and Mark Cuban both agree on this tip

Tony Robbins shares the mindset you need to invest in bitcoin

Digital currency bitcoin is up six times in value for the year, and smashed through records this month. Friday, it hit a new high of $6,000. And, 49 percent of people surveyed by CNBC think it could go even higher, responding that the currency is on the way to hitting $10,000.

If you feel like you're missing out and wondering whether to invest, Mark Cuban and Tony Robbins have some advice: Only invest money you aren't afraid to lose.

"If you're a true adventurer and you really want to throw the Hail Mary, you might take 10 percent [of your savings] and put it in bitcoin or ethereum," Cuban tells Vanity Fair. "But, if you do that, you've got to pretend you've already lost your money."

In addition to sharp gains, bitcoin is also prone to big drops. It lost about $500 in one day earlier this week after indications of greater U.S. regulation, and briefly lost $2,000 in September after China cracked down on digital currencies. The volatility is one reason why critics are skeptical of the crypto-currency.

So Cuban and Robbins say you need to go into it with the mindset that while you hope you could win big, you know you could lose it all.

Tony Robbins
David A. Grogan | CNBC

"I think [bitcoin] is very iffy," Robbins tells CNBC. "I look at that as it's like going to Vegas." As with gambling, you should only bet discretionary funds, he says. Then a win is a big bonus and a loss won't hurt you financially.

When looking at volatile investments like bitcoin, Robbins uses that mentality himself. He explains his thinking is, "I know it is just for fun I'm investing, I know I could lose, this is Vegas."

Cuban says, "It is like collecting art, like collecting baseball cards, like collecting shoes — something is worth what somebody else will pay for it. It is a flier, but I would limit it to 10 percent."

Cuban and Robbins also agree on one of the best long-term investments you can make. They say low-cost index funds are a solid place to put your money.

After saving up six months of income, your next move should be to consider finding "the cheapest SPX mutual fund" you can, Cuban tells Vanity Fair. He's talking about investing in an index fund made up of companies in the S&P 500, which are often low-fee ways to broadly invest in the biggest companies in the stock market.

Robbins likes index funds because, as he writes in his book "Unshakeable," they "take a 'passive' approach." That protects you against "all the downright dumb, mildly misguided or merely unlucky decisions that active fund managers are liable to make," he says.

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