Maybe 3%, if that. And yeah, it definitely depends on your age. I bought a new 2000 Intrepid in the fall of 1999, at the age of 29. That car was $22,389 out the door. My invested assets might have been around $50,000 at best, at the time. So, the car was about 45% of what I had invested. Or, if you counted the car's purchase price into my net worth (which I don't) you could fudge it to about 31%.
Fast forward a few years, to September 2012, and I bought a new, leftover 2012 Dodge Ram, for about $20,750 out the door. Invested assets were around $770,000 by then, so by this time that truck was around 2.7% of that. Or, again if I combined the two, 2.6%.
Now that I see it typed out, I suddenly feel a bit silly balking at the idea of paying $20-30K for a newer car now. Just two months shy of my 53rd birthday, and invested assets around $2.37M. Even $30K would only be around 1.3%. But instead, a few weeks ago I opted to put around $800 into an aging 2003 Regal that's been at death's door for some time now, but just won't ring the bell.
Oh, as for my 3% estimate, that includes the Ram that I still have, the Regal, and a few antiques. But, I don't include the value of the cars in any kind of net worth/invested assets calculations that I make.