For Vanguard's Prime Money Market Fund Investor Shares mutual fund, the minimum investment is $3,000. But for the Admiral Shares version, the minimum investment is $5,000,000! Why is there such a huge difference? It seems like it could be a typo.
Admiral Share class comes with a low expense but higher minimum investment amount. Think of it as a wholesale deal.
From Vanguard's website -
Admiral™ Shares are a separate share class of Vanguard mutual funds that were created to pass along the savings that result from larger accounts to the investors who own them.
Regarding the minimum investment amount, if you look at other prime money market fund and the corresponding "wholesale" version (different institutions use different names: Admiral Shares, Capital Class, Institutional Class etc.), you will see some with even bigger differences. For example, FIPXX requires $10mil.