Why do you need a whole Bitcoin? A Bitcoin is simply a unit. The price of gold is about $38,000,000 per ton. Can you buy a ton of gold? Could you buy a smaller amount of gold?
There are 100,000,000 satoshis per bitcoin. So if 1 BTC = $1M USD then 1 satoshi = $0.01. BTW I personally don't think Bitcoin will reach a price of millions of even hundreds of thousands of dollars per BTC but if it did 8 digits is enough to keep the value of 1 satoshi relative low.