Some prospectors (better referred to as placer miners for the first period of the '49ers) became modestly rich, meaning they obtained enough money (in the low 5 figures) to purchase "that farm back home." Most of the 300,000 or so '49ers who came to California between 1849 and the early 1850s did well enough, but mainly because the Pacific Coast was so abundant in resources and opportunities (and especially non-mining opportunities). But the vast majority did not become millionaires.

Between 1848 and 1860 (a period which includes some hard rock, underground mining as well as the destructive hydraulic mining) roughly $300,000,000 was produced, this at a time when gold sold for roughly $16 per ounce, meaning that the period produced over 1 million pounds of gold. Because much of the period was dominated by small groups of miners, much of this wealth was distributed - in an uneven but a necessarily broad way - among the hundreds of thousands who arrived. See this source for example. And see this collection of gold rush letters which I helped edit for background as to what life, success, and failure was like.

Compare this to the Great Comstock Lode in Nevada - first strike in 1859 with productivity for twenty years (followed by uneven periods of prosperity from mining). During that period roughly $350,000,000 in gold ($16 per ounce) and silver ($1.60 per ounce) was produced. But this was in a small, four mile line of mines as opposed to the California Gold Country that included a great expanse of the western slope of the Sierra. The Comstock produced roughly 500,000 pounds of gold and 5 million pounds of silver. In today's terms, both rushes would have produced sums in the tens of billions (with gold hovering around $1,250 per ounce and silver at about $20 per ounce).

On the Comstock, a few capitalists made most of the money, but laborers did very well, being the highest paid industrial works in the world with a $4 daily minimum for underground work (compare this to 75 cents for canal diggers or $1.25 per day for workers at the Colt Factory at Hartford (see the National Historic Landmark nomination for the Colt district). The California Gold Rush wasn't great for many, but it helped set up some. But as I said, the best thing about the gold rush for most was landing on the West Coast.

For the Comstock, see my book on the subject.