Some things just don't go together. Antarctica and snorkeling. Peanut butter and toothpaste. Cheap stocks and Robinhood investors.

If you look at the list of the 100 most popular stocks held by Robinhood investors, it's loaded with stocks that trade at nosebleed valuations. Not all of them are expensive, though. Here are three Robinhood stocks that are ridiculously cheap.

A tiny shopping cart with $100 bills in it

Image source: Getty Images.

Facebook

Facebook (NASDAQ:FB) ranks as the fifth-largest publicly traded company in the world based on market cap. You might not think that the social media giant is a cheap stock, with shares trading at nearly 27 times expected earnings.

However, when you factor growth prospects into the equation, Facebook is quite inexpensive. The stock's price-to-earnings-to-growth (PEG) ratio stands at a low 1.15. That's well below the PEG ratios for other big-tech stocks.

The popular Robinhood stock recently hit an all-time high, and I suspect it could go much higher. Facebook just launched a new product called Dynamic Ads for Streaming that's sure to be loved by streaming-service providers. When users see an ad in their Facebook and Instagram feeds, they can swipe through the ad to see other titles that they might be interested in based on their interests. 

Over the longer run, Facebook's augmented reality (AR) and virtual reality (VR) investments could be another huge growth driver. The company reportedly has nearly one-fifth of its global workforce focused on AR and VR projects.

Facebook has plans to introduce AR glasses in the not-too-distant future. It's even developing a bracelet that translates electrical activity from motor nerves that will enable users to navigate AR menus by thinking about scrolling.

Moderna

Is Moderna (NASDAQ:MRNA) actually a value stock? There's a pretty good argument to be made that it is

The biotech's market cap of close to $53 billion might seem steep considering that Moderna only recently began selling its first product. However, that product happens to be mRNA-1273, one of the most effective COVID-19 vaccines available.

Moderna expects to make at least $18.4 billion in sales from mRNA-1273 this year, but that figure could be too conservative. Its stock trades at a forward earnings multiple of 7.9. To put that into perspective, the average forward price-to-earnings ratio for biotech stocks is 11, according to Yardeni Research.

Granted, there are plenty of questions about what Moderna's recurring revenue will be after this year. However, with the emergence of new coronavirus variants, there's a pretty good chance that the company should be able to count on a sizable revenue stream from its COVID-19 vaccine. Moderna also has other pipeline candidates that could be winners over the next few years. 

Pfizer

It's probably easier for most investors to accept Pfizer (NYSE:PFE) as a dirt cheap stock. The big-drugmaker's shares trade at a little over 11 times expected earnings, a valuation well below most of its peers.

Pfizer, like Moderna, is popular with Robinhood investors because of its COVID-19 vaccine. However, the pharma stock hasn't exactly benefited all that much from the success of its vaccine: Pfizer's shares are up less than 10% over the last 12 months and are down slightly year to date. I think that could change as the company reports its financial results over the coming quarters. 

Pfizer's joint efforts with partner BioNTech to develop a COVID-19 vaccine could pay off in an even bigger way in the future. The company recently announced that it intends to develop messenger RNA (mRNA) vaccines on its own. This move could allow Pfizer to pocket more profits and establish itself as a top player in its own right in the promising mRNA arena.

Things would be looking up for Pfizer even without its COVID-19 vaccine, though. The company's merged its Upjohn unit with Mylan in November 2020 to create a new entity, Viatris. This deal removed a basket of older drugs that have lost exclusivity from Pfizer's lineup, clearing the way for stronger growth going forward.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.