The upper bound on how many coins he's mined is roughly 1 million (the best estimates I'm aware of come from https://bitslog.wordpress.com ). So you can just interpret the current exchange rate of X as X million. On http://bitcoincharts.com/markets/currency/USD.html I see Bitstamp at $169 right now (MtGox, as pointed out here, is not a useful benchmark anymore), so that's $169m in bitcoins.
(modulo all the usual reasons it's less than $169m: the estimate should be an upper bound and so the real amount lower, he may have lost his bitcoins, you can't cash out that much without crashing the price, he'd have to pay taxes on such an enormous amount, etc etc)
(modulo all the usual reasons it's less than $169m: the estimate should be an upper bound and so the real amount lower, he may have lost his bitcoins, you can't cash out that much without crashing the price, he'd have to pay taxes on such an enormous amount, etc etc)