If you don't have a lot of money to invest, you can still slowly build up your position in some great stocks. Investing $100 per month would total $1,200 over a full year. And if you were to continue to do that for approximately 21 years, you would have invested $25,000. Investing in good, growing businesses can be a great way to make the most of the money you save up and put into stocks.

Three stocks which look unstoppable in the long run and which cost less than $100 include Coca-Cola (NYSE: KO) , DraftKings (NASDAQ: DKNG) , and PayPal (NASDAQ: PYPL) . Here's why these can be great stocks to put in your portfolio for the long haul.

1. Coca-Cola

Coca-Cola may not be a high-powered growth stock, but this is a business that does look unstoppable when you consider how diverse it is and how plentiful its opportunities are. And that's because the company has evolved along with changing consumer tastes.

A good example of that is its popular Coke Zero brand, which caters to people who want to consume less sugar. In its most recent quarter, which ended in June, the company's unit-case volumes grew by 2% year over year. But for Coca-Cola Zero Sugar, the growth rate was 6%.

While North America may seem saturated, the bigger growth opportunities may come in international markets. Latin America, for instance, is an emerging market where the compounded annual growth rate (CAGR) for soft drinks is expected to be as high as 7% until 2027. And that's even higher for categories such as tea, energy, and hot beverages. Coca-Cola has more than 200 brands in its portfolio, which can help it grow and expand and meet that demand. With vast resources, some excellent brands, and profit margins normally in excess of 20%, the company is well equipped to grow its business in the long run.

At around $70 per share, you can easily buy Coca-Cola stock for less than $100 right now.

2. DraftKings

Sports betting has been growing in popularity over the years thanks to more states legalizing it. And a stock that has benefited significantly from those new opportunities is DraftKings, which has a sportsbook and also offers fantasy-sports services.

This year, the company projects that its revenue will come in at around $5.2 billion, which would translate into a year-over-year growth rate of 41%. The company is incurring operating losses, but it has been trending in a positive direction. With a lot more growth still to come for the relatively new industry, it may not be too long before DraftKings is able to consistently post a profit.

Analysts are bullish on the stock's near-term prospects, with a consensus analyst-price target of around $50, which would suggest an upside of more than 60% from where the stock trades today -- approximately $30. In the long run, as the business gets bigger and more states legalize sports betting and as its financials improve, DraftKings' valuation is likely to rise far higher as well.

3. PayPal

Investors have been down on PayPal due to the rising number of competing services the company faces. And while there's no denying that consumers have more ways to transfer money these days than they did even five or 10 years ago, PayPal has built a trusted brand, which still resonates with people today.

The proof is in the results. PayPal reported its most recent earnings numbers last month, and its total-payment volume for the quarter ending June 30 rose by 11% year over year to $416.8 billion. Net revenue of $7.9 billion grew by 8%, and net income of $1.1 billion also increased by 10%.

PayPal's business is still doing well at a time when economic conditions aren't great. And as they improve, the fintech company's growth rate may even accelerate. It has been launching new artificial intelligence-powered products to enhance (i.e., speed up) the checkout process, which could further incentivize shoppers and merchants to use its platform.

At around $65, the stock is not far from its 52-week high of $70.66, but it's still a fairly cheap investment, trading at an estimated 15 times its future earnings.

Before you buy stock in Coca-Cola, consider this: