It will depend on how much money flows in alternative cryptocurrencies. Assuming that Bitcoin's marketshare falls from current 45% to about 25% by 2020 and assuming that the total cryptocurrency asset class grows from 110 billion to about 50 trillion by 2020, I would estimate the market cap of BTC to reach 12.5 trillion by 2020. If the number of BTC by that time is 18 million, the value of one BTC would be around 700.000 usd.

This assumes that the USD maintains a reasonable rate of inflation against basic goods. If the whole financial and monetary system implodes under the weight of the global debt, then BTC would obviously go much higher in USD terms. In real terms, we can anticipate that, following the halving of 2020, the value of one BTC will be that of a very comfortable 3 bedrooms house.

If the marketshare of BTC falls to only 10%, then we would be closer to a value per BTC of 300.000 usd in current usd value.