According to Wikipedia, the Zonda Cinque Roadster is limited to 5 units and costs around $2m, so this is probably a $1.5m-$3m project.
Could DHH have amassed several million in the last few years at 37signals? I can't see why not. Blindly assume they have 100k accounts paying an average of $40 per month (not unreasonable given their price tiers), that's $48m per year. With a typically healthy profit margin, there's no reason why he couldn't be taking a dividend of several million per year.
And with no wife or kids (from what I know), dropping even a quarter of your wealth on a single mindblowing car doesn't strike me as particularly extravagant. After all, some people spend thousands of dollars on a TV when they have barely anything in the bank.
Disclaimer: I have no insider knowledge of DHH's affairs and am just pulling numbers out of my ass that seem possible to me given what DHH and 37signals have said over the years.
In response to a similar question in 2006, Jason Fried said:
We’re a private company so we don’t disclose this information.
A few years ago, someone tried to analyze it and came up with vague revenue estimations: http://blog.jedchristiansen.com/2008/02/25/37signals-is-one-...
My model was largely based on a few key numbers they had published and a general freemium business model. Since then, they've dramatically reduced the prominence of their "free" option and also stopped posting any details of their business, so I haven't been able to realistically update my analysis.
That said, I think that by focusing much more on their paid plans, and by branching out into even more business lines has made 37signals more and more profitable. (See their recent string of new hires.) I wouldn't be surprised if they were exceeding $10million a year in profit, which would be ~$5million a year to DHH. Enough to safely afford a dream car like this. :)