What is a reasonable sum of money to start investing in stocks? $10,000? $1,000? $500? The truth is, thanks to online brokers who now offer trades at low or no fees and fractional shares, investors can get their skin in the game at almost any price.

However, for those who prefer acquiring at least one whole share of a company, there are exciting corporations whose stock prices are well below $30. Let's consider two examples: Exelixis (EXEL -0.65%) and Adyen (ADYE.Y -0.70%). Here's why these stocks are worth buying and holding onto for a while.

1. Exelixis

Exelixis is a biotech that focuses on developing cancer drugs. The company owes most of its revenue to just one medicine, Cabometyx, which treats certain forms of liver and kidney cancer. Cabometyx has been a bit of a "pipeline in a drug," constantly grinding out new indications and delivering stronger revenue. Last year, Exelixis' top line increased by 13.6% year over year to $1.8 billion. The company's earnings per share of $0.65 was roughly 16% higher than the previous fiscal year.

Exelxis' strategy remains the same. The company plans to earn more important label expansions for its crown jewel. It is targeting approval for Cabometyx in treating metastatic castration-resistant prostate cancer. This diagnostic often leaves patients with at most two years to live. It's clearly an area with a high unmet need, which is precisely what Exelixis usually goes after. It has been successful many times with Cabometyx, and given the many clinical trials the medicine is undergoing, we can expect more label expansions.

However, Exelixis is also planning to diversify its lineup. One of the bears' arguments is that it is too reliant on Cabometyx. Its most advanced candidate is zanzalintinib, which is currently undergoing several phase 3 studies, including to treat metastatic colorectal cancer -- another area where there is an unmet need. Patients who catch colon cancer (colorectal cancer is the name given to both colon and rectal cancers) in the early, localized stage have about a 91% five-year survival rate.

Once the disease has spread to distant body parts, such as the liver and lungs, it drops to a paltry 13%. The story is more or less the same with rectal cancer. Beyond Cabometyx and zanzalintinib, Exelixis is running several early-stage clinical trials for a few candidates. The company is also working with other biotechs to discover potential new gems. Exelixis has proven to be a highly innovative company.

Expect the company to continue delivering solid clinical and regulatory progress, robust financial results, and competitive stock market performances. Shares are changing hands for just under $21 apiece, a very affordable price.

2. Adyen

Netherlands-based fintech giant Adyen had a rough go of it last year. The company's revenue growth dropped while margins declined. There were several causes. First, Adyen's own strategy was to blame -- while many of its peers were finding ways to cut expenses, most notably by letting go of thousands of employees, Adyen did the opposite to invest in its future. Second, there is increased competition in its niche.

Adyen provides a platform that allows businesses to accept payments in multiple regions of the world. The company acts as an acquirer (that accepts credit card payments) and a processor and provides point-of-sales systems and risk management services. Other companies, such as PayPal, are increasingly challenging Adyen, which is stifling its progress, especially in the U.S.

Still, there are good reasons to invest in Adyen. In 2023, the company's processed volume reached an impressive 970.1 billion euros (about $1.1 trillion), 26% higher than in 2022. Adyen's revenue increased by 22% year over year to 1.6 billion euros ($1.7 billion). Adyen's top line didn't grow as fast as the market would have liked, but it is still a decent performance for the company.

Meanwhile, Adyen's earnings before interests, taxes, depreciation, and amortization (EBITDA) margin dropped to 46%, compared to 55% in 2022 -- but the good news is that the company is no longer aggressively hiring personnel to invest in the future. That should help boost margins in the mid-term.

Adyen's business benefits from switching costs -- its customers can't easily decide to drop its services and opt for a different payment platform. Doing so takes planning to ensure that the change won't disrupt day-to-day operations or that the new provider will have as broad a range of accepted payment methods as Adyen; otherwise, they risk losing customers.

That's a bit of a headache to go through. It creates a powerful incentive for Adyen's clients not to abandon its services unless necessary. Having a moat is important to a company's long-term success, and it should help Adyen profit from the fast-growing fintech industry. While it has encountered some issues lately, the future still looks bright. Adyen's shares are worth just under $16 at the moment.