A month in the stock market is equivalent to the blink of an eye for Foolish investors. However, for shareholders of Nvidia (NASDAQ: NVDA), it’s long enough to drive noteworthy gains (or losses).

Here, I’ll look at how much I’d have if I’d stuck 10 grand into shares of the artificial intelligence (AI) leader just one month ago. Then consider whether I’d buy the stock right now.

Not a bad monthly return

At the end of September, the Nvidia share price was $121. It’s since gone on to reach $140, which translates into a decent gain of 15.7%. That’s actually more than the FTSE 100‘s managed to muster all year long (with dividends included).

This means my hypothetical £10,000 would now be worth £11,570 on paper.

Nvidia’s become so large that this 15.7% rise in four weeks equals a gain of roughly $470bn (£362bn) in market value. Or the equivalent of Lloyds Banking Group 10 times over!

A big week on Wall Street

The stock could end October even higher because we’ve got crucial quarterly earnings reports from other tech giants this week. I’m talking specifically about the big data centre players.

If these firms all report solid numbers and confirm that AI spending remains a priority, then Nvidia’s share price could spike to a new record. On the other hand, a single cautious sentence on AI from management could spark a sell-off.

Nvidia’s due to report its Q3 2025 earnings on 20 November. The market expects revenue to land somewhere around $32.9bn. That’d represent year-on-year and sequential growth of 81.7% and 9.7% respectively.

Prisoner’s Dilemma

Big Tech’s reportedly set to spend an eye-watering $200bn+ on AI this year, primarily building out infrastructure. But will these firms be able to realise large enough returns to justify this huge expenditure?

In Q2, Alphabet CEO Sundar Pichai admitted: “The risk of under-investing [in AI] is dramatically greater than the risk of over-investing for us here, even in scenarios where if it turns out that we are over-investing.”

Therefore, the risk is that these companies are trapped in a sort of corporate Prisoner’s Dilemma. That is, each one’s spending on AI as a defensive move, driven more by fear of falling behind than by confidence in huge returns. And this cycle of spending might not benefit any of them financially in the end.

Would I buy Nvidia stock?

Scottish Mortgage Investment Trust has been a long-term backer of Nvidia. Indeed, its original 2016 investment in the AI chip pioneer is up more than 85 times in value!

Yet the trust recently reduced its holding, saying: “As we look out over the next five years, there’s a bit more of a symmetric returns potential for Nvidia, than the asymmetry we look for. We still think AI will have huge application, but to do so it will have to be low-cost. So what does that mean for Nvidia’s rapid revenue growth”?

While Nvidia’s undoubtedly world-class, and generative AI may indeed transform every industry one day, investing at the wrong price could also prove costly. At $140, the stock’s price-to-sales ratio is a sky-high 36.

As things stand, I have no plans to buy Nvidia shares. I’d rather invest elsewhere.