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Tesla Motors Inc. Chief Executive Officer Elon Musk Speaks At StartmeupHK Venture Forum
Billionaire Elon Musk, chief executive officer of Tesla Motors Inc., listens during the StartmeupHK Venture Forum in Hong Kong, China, on Tuesday, Jan. 26, 2016. Tesla is looking for a Chinese production partner but is "still trying to figure that out," Musk said. Photographer: Justin Chin/Bloomberg via Getty Images
Bloomberg—Bloomberg via Getty Images

The downside of owing most of your net worth to an erratically profitable company: Your paper value fluctuates. A lot.

After a earnings report that did little to pique investor optimism over the Model 3, shares of Tesla fell, leading to a roughly 7% drop off since Monday.

That made Elon Musk's net worth drop by roughly $800 million over the same period. The billionaire CEO who backs Tesla and aerospace company SpaceX, who owes the majority of his net worth to his electric car company, owned a stake in Tesla worth about $10 billion as of Friday.

His net worth is now nearly $19 billion thanks to his $9.6 billion stake in SpaceX, and excluding some liabilities, based on data from Bloomberg.

Musk's net worth has fluctuated by billions of dollars, as investors have increasingly bet on, or against, Tesla's success. At one point earlier this year, the billionaire was worth nearly $23 billion thanks to optimism surrounding the Model 3.

And both bulls and bears have a case. Tesla has been burning cash in a bid to produce its mass market Model 3. It has only posted two profitable quarters in its history as a company. Yet in a letter to shareholders recently, Tesla warned of Model 3 production bottlenecks. s a result, Tesla said it would reach a rate of producing 5,000 Model 3 vehicles at the start of 2018. Previously, Tesla said it would reach the rate by the end of 2018.

On the other hand, bullish investors are betting on the future of electric vehicles, arguing some patience will result in a successful Model 3 production ramp, leading to steady profitability.

Musk is also in a sense, better off than he was just a year ago. Over the course of the past 12 months, Tesla's stock has risen 59%. According to the Bloomberg Billionaire Index, his net worth was about $8.2 billion back then.